1-Aryl-1-methylethyl N-arylcarbamates and phenyl carbonates and 1-aryl-1-phenylethyl N-p-tolylcarbamates have been prepared and their decomposition kinetics in diphenyl ether, dodecane, and sulpholane studied. Rate constants correlate with σ⁺constants (the value of –0.13 being required for m-CH₃ substituents) giving ρ⁺ values in the range –1.28 to –2.40 at 400–450K. Within one reaction series, ΔS^‡ is not constant, so that ρT= constant does not hold. ΔH^‡ varies linearly with σ⁺ and ρ⁺ values have been corrected for temperature from the slope of ΔH^‡versusσ⁺ which equals 2.3R(ρ₁⁺–ρ₂⁺)T₁T₂/(T₁–T₂). The magnitude of ρ⁺, which depends on solvent, and which is greatest in the carbonate series, indicates a high degree of carbocation formation. Application of Charton's extended Hammett equation shows good correlations with σ_R⁺(although the data sets are limited) and lends support to this view. ²H₆ and ²H₃ substitution in (CH₃)₂C(O₂CNHPh)C₆H₄X (X =m-CH₃, H, or p-CF₃) results in rate retardation arising from both primary and secondary kinetic isotope effects and confirms a polar, but not ionic, reaction mechanism.