An analysis of variational transition state theory rate constants, for the association reactions Cl- + CH3Cl, Cl- + CH3Br, H + CH3, H + diamond{111}, CH3 + CH3, and Al + Al2 shows that the free energy of activation ΔG 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, Δ , and the change in ΔG with temperature, ΔΔG/ΔT. The near linear dependence of ΔG 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 + Al2 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.