The authors investigated the effects of concurrent verbalization on students' performance on a time-critical, dynamic decision-making task. After training on Fire Chief (a computer microworld that simulates fighting a forest fire; M. M. Omodei & A. J. Wearing, 1993a), 60 research participants were allocated randomly to 1 of 3 experimental conditions: silence, associative verbalization, or procedural verbalization. Participants who verbalized the bases of their decisions (procedural verbalization) performed significantly worse on average than participants in the silence condition. There was a small but non-significant decrement in the performance of participants who verbalized thoughts other than the bases of their decisions while performing the task (associative verbalization). Their average level was between the levels of participants in the silence and procedural-verbalization conditions.