Relative values of the total reaction cross-section for the crossed molecular beam K +RI → KI +R(R= CH3, C2H5, C3H7) reactions have been measured at low collision energies. An estimation of the influence of the radical group upon the reaction cross-section was made. AtET= 0·18 eV of collision energy we found 20A2, 30A2and 39A2for the propyl, ethyl and methyl reactions, respectively. These results reveal a larger steric factor with the size of the alkyl group. This is consistent with the (observed) increase in the backward character of the product (KI) centre of mass angular distributions as the reagents' alkyl group size increases.