The combination of diethylpropion hydrochloride with carboxymethylcellulose induces decomposition of the drug. The principal decomposition products obtained were 1-phenyl-1,2-propanedione, benzoic acid and diethylamine as hydrochloride and benzoate salts. These phenomena have been explained with reference to basic decomposition scheme of the diethylpropion. Additionally, the behaviour of diethylpropion in mixtures with other dietary fibres such as methylcellulose, wheat bran and wheat germ was examined. Diethylpropion in combination with methylcellulose is rather stable while in mixtures with wheat bran and wheat germ suffers degradation. An increase in the weight of the excipient, probably due to chemical binding of the drug and other degradation products by the excipient, was observed in all these cases. The decomposition reactions were studied by applying a sequence of selective solvent extractions and using high-performance liquid chromatography (HPLC).