The bidirectional study presented in this thesis contributes to the debate on the influence of the L1 and UG in the process of L2 acquisition by looking at the L2 acquisition of reflexives and reciprocals in Croatian and English. The aim was to determine whether the two factors play a role in acquiring an L2 and if they do, which is more important. The study also aimed to establish whether the Full Transfer/Full Access (Schwartz & Sprouse, 1996) or Modular approach (Montrul, 2000) to L2 acquisition was on the right track. The study was conducted among native Croatian and English speakers who were lower and upper intermediate learners of these languages as the L2s. The participants were given four (three in the case of control groups) tasks—a cloze test, a vocabulary translation task (which was omitted for control groups), a picture judgement task and an acceptability judgement task. The results of the study showed the influence of both factors for L2 Croatian learners, manifested through accepting ungrammatical morphological markings as well as treating unaccusative and unergative verbs as transitive. Given the differences in the amount of errors relating to morphological markings and transitivity rules, the results supported the Modular, rather than the FTFA approach. The findings for L2 English learners are less clear.