Stimuli such as surprised faces are ambiguous in that they are associated with both positive and negative outcomes. Interestingly, people differ reliably in whether they evaluate these and other ambiguous stimuli as positive or negative, and we have argued that a positive evaluation relies in part on a biasing of the appraisal processes via reappraisal. To further test this idea, we conducted two studies to evaluate whether increasing the cognitive accessibility of reappraisal through a brief emotion regulation task would lead (...) to an increase in positive evaluations of ambiguity. Supporting this prediction, we demonstrated that cuing reappraisal, but not in three other forms of emotion regulation (Study 1a-d; n = 120), increased positive evaluations of ambiguous faces. In a sign of robustness, we also found that the effect of reappraisal generalised from ambiguous faces to ambiguous scenes (Study 2; n = 34). Collectively, these findings suggest that reappraisal may play a key role in determining responses to ambiguous stimuli. We discuss these findings in the context of affective flexibility, and suggest that valence bias (i.e. the tendency to evaluate ambiguity more positively or negatively) represents a novel approach to measuring implicit emotion regulation. (shrink)