The congruency effect--that is, faster and more accurate processing of congruent multiword units, has been demonstrated in multiple studies. It is still unclear, however, what its underlying mechanism is, and how congruency may interact with other factors. Using an acceptability judgement task, this study examined the congruency effect in immersive (Experiment 1) and nonimmersive (Experiment 2) L2 learners' collocational processing while taking into account L2 collocation frequency, immersive learners' L2 use, their length and starting age of immersion, nonimmersive learners' length of instruction, and their L2 proficiency. The study also tested whether L1 counterparts of words in L2 collocations were activated. Nonmmersive learners showed a congruency effect in both processing speed and accuracy. In contrast, immersive learners were affected by congruency only in processing accuracy. Higher L2 collocation frequency, greater length of instruction, and higher L2 proficiency did not reduce the congruency effect, whereas longer duration of immersion improved the processing of incongruent items. An effect of L1 lexical frequency was found, an indication of L1 activation. Results were discussed in light of how L2 proficiency and experiences changed the amount of L1 influence in L2 collocational processing.