The reactivity of think-alouds (TAs) (i.e., their effects on thinking) has remained an inevitable concern whenever their use is considered or evaluated. This paper reports findings from a study that involved 85 Chinese sophomores, who, having received some instruction in diction, and then written a narrative in English as a baseline task, completed a similar main writing task, either silently or while thinking aloud. Between-groups differences were analysed in 20 measures of writing performance. TA was found to significantly impair only two measures, lexical diversity and nondysfluencies. TA may have constrained newly developed, still unstable thoughts that were perceived as peripheral, apart from its effects on fluency. Further analyses revealed differential effects. Those with high working memory capacity (WMC), especially high reading span, were most affected in lexical diversity, while those with low WMC experienced a significant decline in organization. The participants' reflections in a follow-up questionnaire, analyzed inductively, show their diverse feelings about the effects of TA, its processing load emerging as the likely primary reactivity-causing factor. Results suggest that, although the reactivity of TAs is small, participants' between- and within-individual differences should be duly attended to in conducting TAs in second language writing and writing research.