Kinetics of the reactions of aryldiazomethanes (ArCHN2) with benzhydrylium ions (Ar2CH+) have been measured photometrically in dichloromethane. The resulting second-order rate constants correlate linearly with the electrophilicities E of the benzhydrylium ions which allowed us to use the correlation lg k = s N(N + E) (eq 1) for determining the nucleophile-specific parameters N and s N of the diazo compounds. UV–vis spectroscopy was analogously employed to measure the rates of the 1,3-dipolar cycloadditions of these aryldiazomethanes with acceptor-substituted ethylenes of known electrophilicities E. The measured rate constants for the reactions of the diazoalkanes with highly electrophilic Michael acceptors (E > −11, for example 2-benzylidene Meldrum’s acid or 1,1-bis­(phenylsulfonyl)­ethylene) agreed with those calculated by eq 1 from the one-bond nucleophilicities N and s N of the diazo compounds and the one-bond electrophilicities of the dipolarophiles, indicating that the incremental approach of eq 1 may also be applied to predict the rates of highly asynchronous cycloadditions. Weaker electrophiles, e.g., methyl acrylate, react faster than calculated from E, N, and s N, and the ratio of experimental to calculated rate constants was suggested to be a measure for the energy of concert ΔG ‡ concert = RT ln­(k 2 exptl/k 2 calcd). Quantum chemical calculations indicated that all products isolated from the reactions of the aryldiazomethanes with acceptor substituted ethylenes (Δ2-pyrazolines, cyclopropanes, and substituted ethylenes) arise from intermediate Δ1-pyrazolines, which are formed through concerted 1,3-dipolar cycloadditions with transition states, in which the C–N bond formation lags behind the C–C bond formation. The Gibbs activation energies for these cycloadditions calculated at the PCM­(UA0,CH2Cl2)/(U)­B3LYP-D3/6-31+G­(d,p) level of theory agree within 5 kJ mol–1 with the experimental numbers showing the suitability of the applied polarizable continuum model (PCM) for considering solvation.