An analysis of variational transition state theory rate constants, for the association reactions Cl- + CH3- Cl, Cl- + CH3Br, H + CH3, H + diamond{111}, CH3 + CH3, and Al + Al-2 shows that the free energy of activation Delta G(double dagger) varies nearly linearly with temperature, over a broad temperature range, for each reaction. This suggests that association rate constants can be parametrized by the free energy of activation at 300 K, Delta G(300)(double dagger) Delta G(double dagger) with temperature, Delta Delta G(double dagger)/Delta T. The near linear dependence of Delta G(double dagger) with temperature is supported by a semiempirical model for association kinetics. The rate constants for Cl- + CH3Cl, Cl- + CH3Br, and CH3 + CH3 decrease with increase in temperature, while those for H + CH3, H + diamond{111}, and Al + Al-2 slightly increase. For the six association reactions considered here, the linear free energy relationship gives semiquantitative rate constants over a rather broad temperature range of 200-2000 K. To calculate accurate rate constants over a broader temperature range, particularly at low temperatures, nonlinear terms must be included in the free energy expansion.