Task-irrelevant stimulus location can influence the response performance to task-relevant attributes, generating the location-based Simon effect. Using a Monte Carlo study and other methods, we examined whether the ex-Gaussian distribution provides a good fit to empirical reaction time (RT) distributions in the Simon task and whether reliable Simon effects occur on the ex-Gaussian parameters: (a) the mean (mu), (b) the standard deviation (sigma) of the normal distribution, and (c) the tail (tau). Results showed that the ex-Gaussian function fits well to empirical RT distributions, and that these ex-Gaussian parameters are reliable between two trial blocks at the group level. At the individual level, correlation analysis showed that the Simon effect was reliable on the mu parameter but not on sigma and tau. Moreover, a partial correlation analysis, with mu s of the two blocks as controlling variables, showed that the Simon effect on tau was reliable. These results provide evidence that the ex-Gaussian function is a valuable tool for analyzing the Simon effect and can be considered as an alternative for analyzing RT distributions in Simon-type tasks.