The rates of barrier-free association reactions are primarily controlled by two features of the potential energy surface-the attraction, V-o(R), between the reactants in their most favorable angular orientation, and the product, IIj(V "(j)(R)), of the force constants, V "(j)(R), for rocking a reactant away from its most favorable orientation at a fixed internuclear distance, R. The product of the rocking force constants in n angular coordinates can be expressed as a quadratic function of the attractive potential, (IIj(V "(j)(R))/R-4)(1/n) = -aV(o) + bV(o)(2). When canonical variational transition-state theory is applied to a potential surface expressed in this form and the rocking motions are treated as classical harmonic oscillators, the rate constant can be expressed as an analytical function of the parameters a and b, of the temperature, and of the average relative velocity of the reactants. The rate constant has a positive activation energy at low temperatures, where the linear term in a dominates; reaches a maximum at a temperature equal to 2a/((eta(2) - 1) bk(B)); and declines at high temperatures, or where the quadratic term above dominates. When applied to the reaction 2CH(3) --> C2H6, the theory underestimates the rate constant at low temperatures but correctly predicts the decline in rate constant at higher temperatures.