Under certain circumstances, it is theoretically important to decide whether a difference between two conditions in mean reaction time (RT) results from a relatively uniform slowing of all responses in the slower condition or from a mixture of some slowed trials with some unslowed ones. This article describes a likelihood ratio test that can be used to differentiate between these two possibilities and reports computer simulations examining the power and Type~I error rate of the test under conditions similar to those encountered in RT research. A freely available computer program called MixTest can be used both to carry out the likelihood ratio test and to conduct simulations evaluating the performance of the test within various settings.