Simple reaction time to stimuli from different sensory modalities presented simultaneously typically is shorter than reaction time to a single stimulus. In this study, auditory, visual, and tactile stimuli were presented in different combinations and at varying stimulus onset asynchronies. Two different types of models for the observed reaction time facilitation effects are developed and tested. Separate activation (race) type models assume that stimulus information in different sensory channels is processed in parallel and independently while coactivation type models allow interactions across different channels. Using Boole's inequality as a test for separate activation models it could be shown that these models cannot predict as much facilitation as observed. A superposition and a diffusion model of coactivation provided a promising quantitative approximation to the data.