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: [(193 ( 65)/T]; k 4 ) (0.759 ( 0.126) × 10 -12 exp[(834 ( 46)/T], and k 5 ) (1.33 ( 0.63) × 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); 3-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.