The stability of USAL (I-L-aspartyl-L-phenylalanine methyl ester hydrochloride) was studied in solutions in relation to the pH value ( I . 10-6.95 at 20 "C), the temperature (20-80 "C at pH 2.2) and the ionic strength (0.16---2.0 at 20 'C and pH 4.0). The effects of pH and temperature of the medium were very significant.From the point of view of the stability the optimum pH value lies in the pH range 2.5-4.5 and at lower temperature. The ionic strengthdid not affect the stability in the tested range. Theeffectsofcitric acid (0-10 7;) and sucrose (0-75 "G) were also investigated. An addition of citric acid exhibited an effect on USAL stability corresponding to the shift of pH value that it caused. The Occurrence of sucrose in the solution had a favorable effect at higher concentrations.Artificial sweeteners based on peptides belong to the recently discovered sweet-tasting substances [l]. It seems that from the whole series of peptides known so far [2], the dipeptidic sweeteners a-L-aspartyl-L-phenylalanine methyl ester (ASPARTAM) and a-L-aspartyl-L-phenylalanine methyl ester hydrochloride (USAL) belong to the most promising for application as sweeteners. They are suitable both for low-energy foods and for diabetics diet. Some properties of ASPARTAM and the possibilities of its application have been already described in the literature [2--51. The stability ofthis sweetener is preimposed by its dipeptidic character. The temperature and the pH of the medium represent the main factors affecting the stability in aqueous solutions. However, in food systems an additional series of factors may exhibit their effects, i. e. the other food components and in some cases the microbiological influence might be expected [6].The stability experiments were performed in model systems, in citrate-phosphate buffers and in the solutions of citric acid and sucrose. The citrate-phosphate buffers very closely approximate by their pH values to the media characteristic for foods. The solutions of citric acid and sucrose form models of fruit sirups that have been used in the production of soft drinks. Experimental