Calculations are reported on the structure of a series of donor-acceptor azobenzenes, (phenylazo)naphthalenes, and phenylazo heterocycles using the AM1 method. The geometries obtained are compared with crystallographic data where available, and reasonable correlations are found between theory and experiment for the bond lengths of the azo and hydrate tautomers of the (phenylazo)naphthols and (phenylazo)-naphthylamines. The calculated heats of formation suggest that the ate tautomers of the azobenzenes and (phenylazo)naphthalenes are more stable than the hydrate form, though the difference between the tautomers of 1-((4’-nitrophenyl)azo)-2-naphthol is only 1.6 kcal mol(-1). The azo tautomer of the phenylazo heterocycles is favored when the donor consists of the NHMe group, but the hydrate form is favored for 2-((4’-nitrophenyl)-azo)-5-hydroxyfuran and -pyrrole. The calculated transition energies of the (phenylazo)naphthalenes reproduce the experimental bathochromic shift observed in moving from the ate tautomer to the hydrate form.