A previous time-series study showed that rapidly repeating a single-word version of a negative self-referential thought reduced the discomfort and the believability associated with that thought. The present parametric study examined whether durations of word repetition were differentially effective in altering the discomfort and believability of negative self-referential thought. In two studies, both discomfort and believability varied systematically with the duration of word repetition. The effects of rapid repetition on emotional discomfort bottomed out after 3 s to 10 s of rapid repetition, whereas the effects on believability did so after 20 s to 30 s of repetition. This study lends support to the cognitive defusion interpretation of the effect of word repetition, suggesting that emotional discomfort and believability may be distinctive functional aspects of cognitive events.