The thermolysis rates of a series of the 1,4-diaryl-2,3-diaza[2.2.1]bicyclohept-2 derivatives 1 with a large variety of aryl substituents (p-NH2, p-OMe, p-Me, H, p-F, p-Cl, p-Br, m-I, p-I, Nt-CN, p-CN, m-NO2, p-NOz, and p-CO(2)Me) have been determined to probe the electronic substituent effects in the thermal decomposition of azoalkanes. The correlation of the logarithmic relative rate constants versus the Creary substituent constants for radical stabilization (cf. Creary, X.; et al. J. Org. Chem. 1987, 52, 3254) improved considerably when polar effects (positive rho value) were included in the form of a two-parameter Hammett treatment. The importance of polar effects was also established in the reported thermolysis rate data for aryl-substituted azopropanes 2, azoethanes 3, azomethanes 4, azoneopentanes 5, and 3,5-diaryl-1-pyrazolines 6. The two-parameter analysis reveals that the thermal decomposition rates of azoalkanes are enhanced by radical- as well as anion-stabilizing substituents. On one hand, the ratio of the radical and polar reaction constants (rho(rad)+rho(pol)) serves as useful parameter to diagnose the radical nature in the decompositions of azoalkanes; thus, in general, radical effects dominate within the azoalkane 1-6 series. On the other hand, the overall sensitivity of the thermal decompositions toward substituent effects is reflected by the sum of the reaction constants (rho(rad)+rho(pol)) The polar effect is attributed to polar destabilization in the ground states of the azoalkanes, for which the polarized C-N bond is weakened by aryl substituents with electron accepters at the positively charged benzylic carbon atom. Semiempirical (AM1) calculations corroborate such polar effects for the azoalkanes 1-6 versus their corresponding diaryl-substituted alkanes 7-12.