Negotiation, as an interactional strategy and proactive focus on form (FoF) have received increased attention in second language research. The combination of negotiation and proactive FoF, however, has not been examined in relation to L2 vocabulary learning. To address this gap, the present study investigated how the amount of negotiation and proactive FoF impacted learners’ vocabulary knowledge development. The participants were 100 adult learners of English, assigned to three experimental groups and one control group. The three experimental groups investigated three categories of no negotiation, limited negotiation, and extended negotiation in proactive FoF. A pretest-posttest design was employed in order to detect any improvement in participants’ vocabulary knowledge components, consisting of collocations, synonyms, derivation, and hyponymy. The results revealed that (a) participants’ vocabulary knowledge significantly improved in all the experimental groups, (b) extended negotiation was significantly superior to limited negotiation, and limited negotiation was significantly superior to no negotiation in proactive FoF, and (c) derivation was the least affected vocabulary component by the treatment. پرونده مقاله