Addressing the lack of studies on L2 online and collaborative prewriting, this study examines L2 collaborative prewriting through voice chat. It explores the effects of an online prewriting condition (individual vs collaborative) on performance in subsequent individual writing as assessed by text length, accuracy, complexity, and analytical rating scales of content, organization, and language. One hundred and twenty-six Thai university students were divided into a control (individual prewriting) group and an experimental (collaborative prewriting) group. The utilised prewriting task was structured, focusing on ideas generation, selection, and organization, and the main task was problem-solution paragraph writing. Independent-sample t-tests revealed that accuracy was comparable across groups; however, the experimental group significantly outperformed the control group on text length, some lexical and syntactic complexity measures, and all rating scores, and the effect size concerning text length was large. Overall, these findings suggest potential benefits of collaboration in voice-chat prewriting.