The thermal degradation kinetics of both components of vitamin C, ascorbic acid (AA) and dehydroascorbic acid (DHAA), were determined in a nectar of Cupuacu (Theobroma grandiflorum) with 25% of pulp and 15% of sugar in water. AA was assayed by HPLC and the results showed that AA degraded into DHAA. A reversible first order model described well the AA degradation data, with an activation energy of 74 +/- 5 kJ/mol and k(80 degrees C) = 0.032 +/- 0.003 min(-1). DHAA kinetic behavior suggested. consecutive first order reaction where DHAA was the intermediate product of AA degradation. A mechanistic model was derived to predict DHAA concentration. Rate constants were replaced by the Arrhenius equation in the model to evaluate the temperature dependence and the kinetic parameters for AA degradation, previously determined, were used. An activation energy of 65 +/- 9 kJ/mol and a k(80 degrees C) of 0.013 +/-0.003 min(-1) were estimated. The present findings will help to predict the best Cupuacu nectar processing conditions that minimize degradation of an important quality factor such as vitamin C. (C) 2000 Elsevier Science Ltd. All rights reserved.