The preferred tautomeric forms of some 2-oxothiazole derivatives were predicted using acidity constants calculated by experimental and ab initio methods. Thiazole derivatives were subject to geometry optimization at two levels of theory: HF/3-21G and B3LYP/6-31G(d). It was observed that oxo forms were favored. An excellent correlation between experimental and ab initio acidity constant values for non-tautomeric molecules was obtained. (c) 2004 Elsevier B.V. All rights reserved.