For various mechanisms of group-transfer polymerization (GTP), the concentration of the active chain ends is calculated. The calculated reaction orders with respect to catalyst and initiator concentrations, respectively, are compared to experimental data for the GTP of methyl methacrylate in THF catalyzed by both bifluoride and benzoate and of n-butyl acrylate in toluene catalyzed by mercury iodide. The comparison reveals that for catalysis by benzoate and mercury iodide, the experimental reaction orders are only consistent with the associative mechanism of GTP, i.e., with an activated silyl ketene acetal as the active species. For the catalysis with the less nucleophilic benzoate anion, the existence of a dissociative mechanism (with enolate ions or ion pairs as the active species) can be neither excluded nor proven. The effect of silyl esters which have been used in "livingness enhancers" is discussed and quantified in terms of all mechanisms discussed.