The nucleophile-specific parameters N and s_N, as defined by log k_20°C=s_N(N+E) have been derived for the guanidines 1 a–h from the second-order rate constants of their reactions with diarylcarbenium tetrafluoroborates in CH₂Cl₂ at 20°C. The applicability of these parameters for predicting rate constants of the reactions of guanidines with ordinary Michael acceptors has been demonstrated. Comparison with other organocatalysts shows that 1,5,7-triazabicyclo[4.4.0]dec-5-ene, the strongest nucleophile of this series, exceeds the nucleophilicity of diazabicyclononene and 4-(dimethylamino)pyridine by factors of 3 to 7.