Relative rate coefficient data have been obtained for the reactions Br ϩ RCHO : RCO ϩ HBr for a series of aldehydes: HCHO, reaction (1); CH 3 CHO, reaction (2); CH 3 CH 2 CHO, reaction (3); CH 3 CH 2 CH 2 CHO, reaction (4). Measurements were made over the temperature range 240-300 K in an environmental chamber/FTIR spectrometer system, using standard relative rate techniques. All measured rate coefficient ratios were found to be independent of temperature over the range studied (k 2 /k 1 ϭ 3.60 Ϯ 0.29, k 3 /k 1 ϭ 6.65 Ϯ 0.53, k 4 /k 1 ϭ 8.62 Ϯ 0.69, and k 3 /k 2 ϭ 1.80 Ϯ 0.14), implying that the activation barriers for all four reactions are essentially identical with the A-factors increasing with the size of the aldehyde. Relative rate coefficients for k 1 and k 2 agree well with currently recommended data at room temperature, but inconsistencies on the order of 20% arise at lower temperatures. The entire set of relative rate coefficient measurements is put on an absolute scale using a combination of currently recommended values for k 1 and k 2 . The following expressions (all in units of cm 3 molecule Ϫ1 s Ϫ1 ) are obtained: k 1 ϭ (0.79 Ϯ 0.10) ϫ 10 Ϫ11 exp(Ϫ580 Ϯ 200/T), k 2 ϭ (2.7 Ϯ 0.4) ϫ 10 Ϫ11 exp(Ϫ567 Ϯ 200/T), k 3 ϭ (5.75 Ϯ 0.75) ϫ 10 Ϫ11 exp(Ϫ610 Ϯ 200/T), k 4 ϭ (5.75 Ϯ 0.75) ϫ 10 Ϫ11 exp(Ϫ540 Ϯ 200/T), where uncertainties quoted for the A-factor reflect the uncertainty in the room temperature value.