A surfactant-induced and temperature conformational transition of catalase has been examined by difference spectroscopy for n-decyltrimethylammonium bromide (C10TAB) in different media (pHs 3.2, 6.4 and 10.0). The conformational transition of catalase by C10TAB was followed as a function of denaturant concentration using absorbance measurements at 280 nm and the data were analysed to obtain the Gibbs energy of the transition in water (ΔG0w) and in a hydrophobic environment (ΔG0hc) for saturated protein–surfactant complexes. Changes in absorbance at 280 nm of catalase with temperature were used to determine the parameters characterising the thermodynamics of unfolding, melting temperature (Tm), enthalpy (ΔHm), entropy (ΔSm) and heat capacity (ΔCp). The results suggest that catalase interacts with C10TAB in the media of pHs 6.4 and 10.0 inducing a conformational transition. However, an interaction in acid medium of pH 3.2 was not present. Finally, we compared the interaction of catalase with other cationic surfactants studied by other authors.