Using a relative rate method, rate constants have been measured at 296 +/- 2 K for the gas-phase reactions of the OH radical with 1-hydroxy-2-butanone, 3-hydroxy-2-butanone, 1-hydroxy-3-butanone, 1-hydroxy-2-methyl- 3-butanone, 3-hydroxy-3-methyl-2-butanone, and 4-hydroxy-3-hexanone, with rate constants tin units of 10(-12) cm(3) molecule(-1) s(-1)) of 7.7 +/- 1.7, 10.3 +/- 2.2, 8.1 +/- 1.8, 16.2 +/- 3.4, 0.94 +/- 0.37, and 15.1 +/-3.1, respectively, where the error limits include the estimated overall uncertainty in the rate constant for the reference compound. Rate constants were also measured for reactions with NO3 radicals and O-3 Rate constants for the NO3 radical reactions tin units of 10(-16) cm(3) molecule(-1) s(-1)) were 1-hydroxy-2-butanone, <9; 3-hydroxy-2-butanune, 6.5 +/- 2.2; 1-hydroxy-3-butanone, 122; 1-hydroxy-2-methyl-3-butanone, <22; 3-hydroxy-3-methyl-2-butanone, <2; and 4-hydroxy-3-hexanone, 12 +/- 4, where the error limits include the estimated overall uncertainties in the rate constants for the reference compounds. No reactions with O-3 were observed, and upper limits to the rate constants of <1.1 x 10(-19) cm(3) molecule(-1) s(-1) were derived for all six hydroxycarbonyls. The dominant tropospheric loss process for the hydroxycarbonyls studied here is calculated to be by reaction with the OH radical.