The chemical stability of a novel cysteine chloromethyl ketone derivative (HI-131) with anti-leukemic activity has been investigated in a microemulsion formulation. HI-131 degrades to two major products, most likely by undergoing oxidation and further reaction with another HI-131 molecule to form higher molecular weight oligomers of the original compound. The degradation kinetics of HI-131 have been studied as a function of pH, buffer composition, ionic strength, and temperature. Degradation follows pseudo-first-order kinetics and the temperature effect obeys the Arrhenius equation. The pH-rate profile demonstrates HI-131 is most stable at lower pH values, although there is no significant influence of ionic strength and buffer ions on the degradation rate. The chemical stability of a homologous series of chloromethyl and diazomethyl ketone derivatives of HI-131 has also been investigated in microemulsion. The relationship between the chain length of the derivatives and the stability is presented. Changing the chloro group to a bromo group resulted in an increase in degradation rate. Alterations to the group on the nitrogen were also investigated. The changes to the stability are discussed in terms of their mechanistic implications.