The structure and relative stability of ground-state isomeric forms and corresponding transition states for thermal cis-to-trans isomerization of a series of N-(phenylazo)-substituted nitrogen heterocycles were calculated, in cyclohexane and dimethyl-sulfoxide, using density functional theory combined with the polarizable continuum solvation model. Bond length trends and solvent-induced geometry changes are rationalized in terms of the degree of π-donation from N-heterocycle to phenylazo moiety. Isomerization is predicted to proceed by a rotation mechanism via a zwitterionic transition state structure. Computed Gibbs energies of activation reproduce qualitatively experimental reactivity trends previously reported; resulting rate constants, however, are ca. 2 to 57 times higher than corresponding experimental values.