In this study, the UV absorbance of oxalic acid that change with temperature and concentration in the dissociation media was investigated. For every solution system, the relationships between absorbance, A, and concentration, c, were obtained at different temperatures. Molar extinction constants of the solutions were calculated by using the Lambert-Beer law. The results indicated the invalidity of Lambert-Beer's law at high concentrations. The temperature dependence of the molar extinction constant of each solution was determined. (c) 2005 Elsevier B.V. All rights reserved.