Dimethyl methylphosphonate (DMMP) and diethyl methylphosphonate (DEMP) were used as models for hazardous organophosphorus compounds to study their radiolytic reactions. Hydrated electrons and superoxide anion radicals do not appreciably react with the phosphonates. On the other hand, hydroxyl radicals degrade the model compounds and the associated rate constants were determined by competition kinetics with thiocyanate to be (2 +/- 1) x 10(8) M-1 s(-1) for DMMP and (6 +/- 1) x 10(8) M-1 s(-1) for DEMP. The initial products formed from an (OH)-O-. radical reaction with the phosphonates are mainly carbon-centered radicals created by H-abstraction. The initial products readily react with TNM to yield adducts, with characteristic absorbances at similar to 300 nm, with rate constants of (8.8 +/- 0.3) x 10(7) and (3.5 +/- 0.3) x 10(8) M-1 s(-1) for DMMP and DEMP, respectively. These hydrogen abstraction products react with oxygen to give the phosphonic acid monoesters at G values of 5.1 (5.3 x 10(-7) mol J(-1)) for methyl methylphosphonate and 4.7 (4.9 x 10(-7) mol J(-1)) for ethyl methylphosphonate. Thus, the hydrogen abstraction pathway constitutes a major route for (OH)-O-. radical reactions, which is also supported by the absence of electron abstraction and/or (OH)-O-. addition reactions.