This longitudinal study investigates the effect of mode of reading on the incidental learning of collocations and factors that affect learning. One hundred Vietnamese pre-intermediate learners of English as a foreign language (EFL) were assigned to either an experimental group or a control group (no treatment). In 9 weeks, the experimental group read three graded readers containing 32 target collocations in three counterbalanced reading modes: reading-only, reading-while-listening, and reading with textual input enhancement (i.e., underlining). Learning gains were measured by a form recall pretest and delayed posttest. The results showed that reading mode had a significant effect on incidental collocation learning. Reading with textual input enhancement resulted in significantly higher learning gains than the other reading modes. Reading-while-listening was also more beneficial for collocation learning than reading-only. Learners' prior vocabulary knowledge and congruency of collocations were significant predictors of the learning gains.