Rate constants for the reactions of OH radicals and NO3 radicals with O,O-diethyl methylphosphonothioate [( C2H5O)(2)P( S) CH3; DEMPT] and O,O,O-triethyl phosphorothioate [( C2H5O)(3)PS; TEPT] have been measured using relative rate methods at atmospheric pressure of air over the temperature range 296-348 K for the OH radical reactions and at 296 ( 2 K for the NO3 radical reactions. At 296 ( 2 K, the rate constants obtained for the OH radical reactions ( in units of 10(-11) cm(3) molecule(-1) s(-1)) were 20.4 +/- 0.8 and 7.92 +/- 0.27 for DEMPT and TEPT, respectively, and those for the NO3 radical reactions ( in units of 10(-15) cm(3) molecule(-1) s(-1)) were 2.01 +/- 0.20 and 1.03 +/- 0.10, respectively. Upper limits to the rate constants for the reactions of O-3 with DEMPT and TEPT of < 6 x 10(-20) cm(3) molecule(-1) s(-1) were determined in each case. Rate constants for the OH radical reactions, measured relative to k( OH + alpha-pinene)) 1.21 x 10(-11) e(436/T) cm(3) molecule(-1) s(-1), resulted in the Arrhenius expressions k( OH + DEMPT)) 1.08 x 10(-11) e(( 871 +/- 25))/T cm(3) molecule(-1) s(-1) and k( OH + TEPT)) 8.21 x 10(-13) e(( 1353 +/-49))/T cm(3) molecule(-1) s(-1) over the temperature range 296-348 K, where the indicated errors are two least-squares standard deviations and do not include the uncertainties in the reference rate constant. Diethyl methylphosphonate was identified and quantified from the OH radical and NO3 radical reactions with DEMPT, with formation yields of 21 +/- 4%, independent of temperature, from the OH radical reaction and 62 +/- 11% from the NO3 radical reaction at 296 +/- 2 K. Similarly, triethyl phosphate was identified and quantified from the OH radical and NO3 radical reactions with TEPT, with formation yields of 56 +/-9%, independent of temperature, from the OH radical reaction and 78 +/-15% from the NO3 radical reaction at 296 +/- 2 K.