When presented with auditory, visual, or bimodal audiovisual stimuli in a speeded detection/discrimination task, participants fail to respond to the auditory component of the bimodal targets significantly more often than they fail to respond to the visual component. Signal detection theory (SDT) was used to explore the contributions of perceptual (sensitivity shifts) and decisional (shifts in response criteria) factors to this effect, known as the Colavita visual dominance effect. Participants performed a version of the Colavita task that had been modified to allow for SDT analyses. The participants had to detect auditory and visual targets (presented unimodally or bimodally) at their individually determined 75% detection thresholds. The results showed a significant decrease in participants' sensitivity to auditory stimuli when presented concurrently with visual stimuli (in the absence of any significant change in their response criterion), suggesting that Colavita visual dominance does not simply reflect a decisional effect, but can be explained, at least in part, as a truly perceptual phenomenon. The decrease in sensitivity (to auditory stimuli) may be attributable to the exogenous capture of participants' attention by the visual component of the bimodal target, thus leaving fewer attentional resources for the processing of the auditory stimulus. The reduction in auditory sensitivity reported here may be considered an example of crossmodal masking.