Rate coefficients have been measured for Cl atom reactions under ambient conditions with acetone and four cyclic ketones. Cl was generated by UV photolysis of Cl2, and other species were monitored by FT‐IR spectroscopy. The measurements yield k(Cl + acetone) = (2.0 ± 0.7) × 10−12, k(Cl + cyclobutanone) = (10.1 ± 0.8) × 10−11, k(Cl + cycloheptanone) = (24.0 ± 2.3) × 10−11, k(Cl + 2‐methyl cyclopentanone) = (15.2 ± 1.2) × 10−11, and k(Cl + 2‐methyl cyclohexanone) = (11.2 ± 1.0) × 10−11 cm3 molecule−1 s−1, where the uncertainties represent 95% confidence limits. These results are discussed in the context of structure‐activity relationships. We also present a prediction for Cl + cyclopropanone based on ab initio properties of the transition state.