The aim of this study was to assess the contribution of sociodemographic data, personality traits, self-esteem and emotional intelligence to the explanation of loneliness in high school students and to assess age and gender differences in emotional intelligence, self-esteem and loneliness. The study was conducted on a sample of N=193 students of first and third grade of Trading and Commercial school Davor Milas in Osijek. The instruments used in the research are: sociodemographic questionnaire, Emotional Competence Questionnaire, University of California Los Angeles Loneliness Scale, Emotion management test, Rosenberg Self-Esteem Scale and Big Five inventory. Analysis of gender and age differences indicates that there are statistically significant differences in loneliness. It was found that girls reported higher levels of loneliness compared to boys as well as third grade students compared to first grade students. The results obtained on a sample of adolescent boys show that extraversion and self-assessment of emotional intelligence are significant negative predictors of loneliness. While the results obtained on a girl’s sample show that extraversion and agreeableness are significant negative predictors. Furthermore, there was a statistically significant difference in self-esteem with regard to gender. Boys have more self-esteem than girls do. The results obtained in Emotional Competence Questionnaire show that there are a statistically significant differences in age. First grade students have higher scores than third grade students. Finally, a statistically significant difference was found in the results on Emotion management test with regard to gender. Girls have higher scores compared to boys.