Absolute rate constants are reported for the addition of the methyl radical to 20 monosubstituted and 1,1-disubstituted alkenes at room temperature in liquid solution. They increase with increasing exothermicity of the addition and are enhanced by strong nucleophilic polar effects for electron-deficient alkenes. This agrees with conclusions derived from earlier relative rate data but disagrees with a general insignificance of polar effects predicted by ab initio calculations.