||Three groups of alkyl and phenyl substituted naphthazarins were studied by ('1)H and ('13)C NMR in chloroform-d at ambient temperatures. ('1)H NMR studies were performed on selected members of each group at low temperatures (to -120(DEGREES)C) in methylene chloride-d(,2) and pentane-d(,12) solutions. Monoalkyl and phenyl naphthazarins were found to exist predominantly as the 2-alkyl tautomers, and the dimethyl alkyl and phenyl species as the 2,3-dimethyl-6-alkyl and phenyl tautomers. However, both symmetrically and unsymmetrically 2,6- and 2,7-disubstituted naphthazarins exhibited averaged chemical shifts and coupling constants indicating rapid proton exchange between the two minimum energy 1,4-dione tautomers a and a'. ('13)C NMR shifts have been utilized to calculate tautomer populations of the unsymmetrical 2,6- and 2,7-(methyl, alkyl) naphthazarins at ambient temperature. The ambient temperature tautomer populations and spin simulations along with low temperature ('1)H NMR gave tautomer populations as a function of temperature.