Among naphthoquinones, lapachol and many heterocyclic derivatives have been investigated during the past years, mainly due to their antibacterial, antifungal and anticancer activities. Assessment of possible incompatibility between an active component (i.e. lapachol) and different excipients along with the evaluation of thermal stability are crucial parts of a normal study prior to the final formulation setting of a medicine. DSC study was used as an important and complementary tool during pre-formulation to determine the compatibility of drug-excipients with the purpose of developing a lapachol gel-cream formulation. The DSC curves of lapachol (Lpch) and binary mixtures with excipients (propylparaben, methylparaben, edetic acid, isodecyl oleate, trietanolamine, glyceril monoesterate, cetiol, glicerin, carbopol, ceromacrogol and mineral oil) were obtained. The results showed that Lpch only exhibited interaction which could influence the stability of the product in the binary mixtures of Lpch/methylparaben, Lpch/cetostearyl alcohol and Lpch/glyceryl monostearate. There was a significant shift or absence of lapachol melting peak in both cases. Photovisual DSC was used to confirm the results obtained with conventional DSC because it is a more sensitive method. (C) 2007 Elsevier B.V. All rights reserved.