Absolute rate constants have been measured for the gas-phase reactions of hydroxyl radicals with a series of ketones : acetone (k(1)), 2-butanone (k(2)), 3-methyl 2-butanone (k(3)), 4-methyl 2-pentanone (k(4)), and 5-methyl 2-hexanone (k(5)) Experiments were carried out using the pulsed laser photolysis-laser-induced fluorescence technique over the temperature range 243-372 K. The obtained kinetic data were used to derive the Arrhenius expressions : k(1) = (1.25 +/- 0.22) x 10(-12) exp[-(561 +/- 57)/T]; k(2) = (1.19 +/- 0.18) x 10(-12) exp[-(60 +/- 61)/T]; k(3) = (1.58 +/- 0.35) x 10(-12) exp[(193 +/- 65)/T]; k(4 ) = (0.759 +/- 0.126) x 10(-12) exp[(834 +/- 46)/T], and k(5) = (1.33 +/- 0.63) x 10(-12) exp[(649 +/- 140)/T] (in units of cm(3) molecule(-1) s(-1)). At room temperature, the rate constants obtained (in units of 10(-12 )cm(3) molecule(-1) s(-1)) were as follows : acetone (0.184 +/- 0.024); 2-butanone (1.19 +/- 0.18); 5-methyl 2-butanone (2.87 +/- 0.29); 4-methyl 2-pentanone (12.1 +/- 0.9); and 5-methyl 2-hexanone (10.3 +/- 1.0). Our results are compared with the previous determinations and discussed in terms of structure-reactivity relationships.