Carbon-13 NMR is used to study the tautomeric equilibrium of citrinin in the solid state. The results confirm earlier X-ray diffraction studies, which indicated that the compound crystallizes in a disordered structure, with the p-quinone and o-quinone forms in a dynamic equilibrium. Analysis of the NMR data yields the equilibrium constant K(T) = [o]/[p] = exp(4.2/R) exp(-1610/RT) (where R is in cal mol(-1) K-1), which is essentially identical to that determined by X-ray. The tautomerism is extremely fast on the NMR time scale (>10(6) s(-1)). In methylene chloride solution citrinin also exists as a fast interconverting mixture of the two isomers with an equilibrium constant K similar to 0.7 at room temperature. However, the temperature dependence of the carbon-13 and oxygen-17 chemical shifts gave conflicting results, thus preventing a reliable determination of the thermodynamic parameters of K. In methanol and methanol/methylene chloride mixtures, citrinin undergoes a nucleophilic, Michael type, addition. The reaction is reversible, and the equilibrium shifts toward the normal citrinin form upon increasing temperature and in methanol/methylene chloride mixtures with increasing methylene chloride fraction. Only normal citrinin is obtained on crystallization, even from neat methanol.