The nucleophilicity parameters N and sɴ, as defined by the linear free-energy equation log k(20 °C) = sɴ (N + E), of the 2-imidazolines and the related N-heterocyclic compounds have been determined by studying the rates of their reactions with differently substituted benzhydrylium ions in dichloromethane at 20 °C by stopped-flow or laser flash photolysis techniques. It is demonstrated that the N and sɴ parameters thus obtained can be used to reliably predict the rate constants for their reactions with Michael acceptors of known electrophilicity E. A comparison of the nucleophilicity parameters of the imidazoline derivatives with other commonly used nucleophilic organocatalysts shows that they are 10 to 10³ times less nucleophilic than PPh₃, 1,8-diazabicyclo[5.4.0]undec-7-ene, or 4-(dimethylamino)pyridine. The structure–reactivity relationships of these heterocycles are discussed.