The effects of micelles of cetyltrimethylammonium bromide (CTAB) and cetylpyridinium bromide (CPB) on the observed pseudo-first-order rate constants for the interaction of ninhydrin with tryptophan were studied. The influence of different parameters was considered, i.e., reactant concentration, surfactant concentration, temperature, and effect of added salts. The data are explained in terms of the pseudo-phase model of the micelles.