Equations have been derived for the pre-exponential rate factors of abstraction reactions. The activated complex is considered as made up of three rigid groups. Classical partition functions are used and it is shown that the use of classical methods gives a simple expression which requires only a knowledge of several force-constants and bond lengths; thus the need for detailed structural information on reactants and complex and for calculations of moments of inertia and vibrational frequencies is eliminated. It is shown that quantum corrections need not be considered in many cases, although if desired, they may be estimated from a graph provided. The method is applied to four hydrogen abstraction reactions and to the reaction, CCl3Br+Br=CCl3+Br2. Satisfactory agreement is obtained.