Based on a prior study by Chen and Truscott, the present study investigated the possible effects of repetition (repeated exposure) and L1 lexicalization on the incidental acquisition and retention of 10 English target words by 90 Persian-speaking EFL learners at an Iranian university. Seven aspects of vocabulary knowledge were measured, including receptive and productive knowledge of orthography, parts of speech, association and meaning. Results showed that, in general, increasing the number of exposures to target words (from one to three or seven) had a positive effect on incidental acquisition. However, there were significant differences in the gains observed for different aspects of vocabulary knowledge both immediately and after two weeks. Non-lexicalized words also appeared to cause learning difficulty mainly in semantic aspects of vocabulary knowledge. The findings of this study are discussed and pedagogical implications are highlighted for language teachers and learners.