The structure of negative-working diazo resins has been investigated by means of quantum mechanical modeling of models for the polymerization processes leading to their formation. Comparison of the predicted heats of formation, the ultraviolet-visible (UV-Vis) spectra, and the infrared spectra with experimental results leads to the conclusion that the diazonium structure of 4-diazodiphenylamines is most stable in acidic media. The regioselectivity for the condensation polymerization reaction of such species with O-protonated formaldehyde cation was examined using atomic charge ( electrostatic potential ( ESP)), frontier molecular orbital, and product development approaches. Although methylol substitution at both rings seems possible, a reaction that leads to ortho- and para-substitution of the non-diazo-containing ring (beta-ring) seems to be favored, according to product development and frontier orbital considerations. During subsequent polymerization, similar conclusions are reached regarding the electrophillic attack on 4-diazodiphenylamine by diazo carbocations derived from dehydration of 4-diazodiphenylamine methylol derivatives. In contrast, atomic charge considerations suggest the feasibility of the reaction involving the alpha-ring.