It is tempting to blame the discrepancy between theory and experiment on calculation of TΔS, especially for the aluminoxanes, which have disproportionately more low energy vibrations that make a large contribution to the vibrational entropy. However, TΔS ‡ -qh-tr values vary by ±3σ = 6.0 kJ mol - 1 including those for the boron-based activators (Table ). There is a larger variation seen in uncorrected TΔS ‡ -tr values while the largest deviation between TΔS ‡ -qh-tr and TΔS ‡ -tr is seen for the aluminoxanes with TΔΔS ‡ = 7.1 ± 3.8 kJ mol -1 . There is a weak correlation between TΔS ‡ -qh-tr or TΔS ‡ -tr and size for the aluminoxanes. This suggests that TΔS ‡ either scales with molecular size or is being over-estimated for the larger aluminoxanes but by ≤ 6.0 kJ mol -1 . Thus, our calculated barriers may be in error for the aluminoxanes but not to the degree where these would be as reactive as borane or borate.