This study examined effects of synonym generation on second language (L2) vocabulary learning during reading in both incidental and intentional vocabulary learning contexts. Spanish-speaking adult learners of L2 English (N = 114) at low- and high-intermediate proficiency levels read an English passage containing 10 target words translated in the text. Participants were assigned to one of four conditions: (a) Read for meaning only ("incidental"). (b) Read for meaning and try to learn the translated words ("intentional"). (c) Read for meaning and generate Spanish synonyms for the translated words ("incidental + semantic"). (d) Read for meaning, try to learn the 10 translated words, and generate Spanish synonyms for the translated words ("intentional + semantic"). Posttest measures were English-to-Spanish and Spanish-to-English recall of target words. Target word recall was higher when explicit instructions to learn new words were provided and when synonym generation was not required. Negative effects of synonym generation emerged in both the incidental and intentional learning conditions. (Contains 5 tables and 4 figures.)