Rate constants for the gas-phase reactions of the cyclic organosulfur compounds 1,4-thioxane and 1,4-dithiane with NO3 radicals and O-3 have been measured at 296 2 K, and rate constants for their reactions with OH radicals have been measured over the temperature range 278-350 K. Relative rate methods were used to measure rate constants for the OH radical and NO3 radical reactions. The OH radical reaction in the presence of NOx and, to a lesser extent, the NO3 radical reaction were subject to secondary reactions leading to additional removal of 1,4-thioxane and/or 1,4-dithiane. The rate constants obtained for the NO3 radical and O-3 reactions at 296 +/- 2 K were (5.1 +/- 1.1) x 10(-14) cm(3) molecule(-1) s(-1) and <2 x 10(-19) cm(3) molecule(-1) s(-1), respectively, for 1,4-thioxane and (5.9 +/- 1.8) x 10(-14) cm(3) molecule(-1) s(-1) and <2.5 x 10(-19) cm(3) molecule(-1) s(-1), respectively, for 1,4-dithiane. For the OH radical reactions, the temperature-dependent rate expressions obtained were k(OH + 1,4-thioxane) = 2.54 x 10(-12) e((619 +/- 51)/T) cm(3) molecule(-1) s(-1) (278-349 K) and k(OH + 1,4-dithiane) = 3.71 x 10(-12) e((621 +/-163)/T) cm(3) molecule(-1) s(-1) (278-350 K), with 298 K rate constants of (2.03 +/- 0.41) x 10(-11) cm(3) molecule(-1) s(-1) for 1,4-thioxane and (2.98 +/- 0.75) x 10(-11) cm(3) molecule(-1) s(-1) for 1,4-dithiane. For the experimental conditions employed, aerosol formation from the OH radical-initiated reactions of both 1,4-thioxane and 1,4-dithiane was important, accounting for similar to 60% of the organosulfur compounds reacted in both the presence and absence of NOx. The data obtained here for 1,4-thioxane and 1,4-dithiane are compared with literature data for the corresponding reactions of simple acyclic alkyl sulfides and ethers.