Relative acidities and basicities of a series of heterocyclic nuclei have been evaluated with the help of absorption maxima data of the following groups of compounds: (i) Dimethin merocyanines and their aza analogues. (ii) p-Dialkylaminobenzylidene derivatives of ketomethylene compounds and their aza analogues. (iii) p-Dialkylaminostyryl dyes and their aza analogues. (iv) Unsymmetrical cyanines and their aza analogues