Self-Determination Theory presents a macro-theory of human motivation that has been applied to many risky behaviours. Nevertheless, there is a scarce literature on the role of this theory in predicting alcohol and cannabis consumption. The aim of this research was to determine how well some constructs of Self-Determination Theory predict frequency of alcohol and cannabis consumption among the first and the second year students at the University of Zagreb that lived in student dormitories. Three instruments were translated from English to Croatian: Self-Determination Scale (Sheldon & Deci, 1993), General Causality Orientations Scale (Deci & Ryan, 1985) and The Learning Climate Questionnaire (Williams & Deci, 1996). A pilot study was conducted on a convenient sample of 114 (32.5% males and 67.5% females) participants of the first and the second year of the study at the University of Zagreb that lived in student dormitories. Based on the data from the pilot study, the factor structure of the instruments was examined, and General Causality Orientations Scale was shortened and adapted. The main research was conducted on a quota sample of 438 (37.9% males and 62.1% females) participants of the first and the second year of the study at University of Zagreb that lived in student dormitories. While planning the sample, efforts were made to ensure that ratio of participants regarding the gender and the field of the study represents population ratios. In the research the following instruments were applied: Self-Determination Scale (Sheldon & Deci, 1993); adapted version of the autonomy and control subscales of the General Causality Orientations Scale (Deci & Ryan, 1985); autonomy subscale of the General Need Satisfaction Scale (Gagné, 2003); The Learning Climate Questionnaire (Williams & Deci, 1996); extraversion, conscientiousness and neuroticism subscales of the International Personality Item Pool (IPIP50); and the questionnaire on the frequency of alcohol and cannabis consumption (adapted version of the European Model Questionnaire, EMCDDA, 2002; questions on synthetic cannabinoids were also included). Separate hierarchical regression analysis for males and females were conducted with a number of days in the last 30 days that participant consumed alcohol, and the number of days participant consumed cannabis in a lifetime as criteria. In predicting alcohol consumption among males, the proposed model explained 13.1% of the variance of this behaviour, and extraversion showed to be a significant predictor. Among females, the model explained 10.9% of variance of alcohol consumption, with extraversion and autonomy orientation as significant determinants. The proposed model failed to explain cannabis consumption among males; there were no significant predictors. Among females, the model explained 11.4% of the variance of cannabis consumption. Significant predictors were extraversion, autonomy causality orientation, and need for autonomy. In the total sample, moderation effects of self-determination and personality traits did not occur, while extraversion and conscientiousness were significant, although weak, predictors of alcohol consumption. On the other hand, there is significant interaction effect of self-determination and extraversion on the frequency of cannabis consumption. The whole model explained 5.4% of variance of cannabis consumption, out of which the interaction effect explained 3.1% of variance. Among participants that were average and high in self-determination, increase in extraversion was followed with an increase in cannabis consumption, and this effect was stronger among students with higher self-determination. The effect was not present among students with low self-determination. Among these participants, cannabis consumption was the same, regardless their extraversion. Since alcohol and cannabis consumption are complex phenomena, it would be unrealistic to expect that high proportion of variance could be explained with motivational theory. In spite of that, this research gave important contributions in understanding alcohol and cannabis consumption among students that live in student dormitories and are at the beginning of their studies. Implications are also recognized on scientific and practical levels. While planning prevention interventions, the fact that different predictors determine alcohol and cannabis consumption among males and females should be taken into an account, as well as the result that students that seem to function well, are not necessarily sufficiently equipped for life challenges.