OBJECTIVE — To use isothermal calorimetry to measure the degradation of reconstituted busulfan at a number of elevated temperatures and to use these data to determine whether the shelf-life of the product could safely be increased from its current period of 15h. METHOD — Busilvex (60mg busulfan in 10ml non-aqueous solvent) was diluted to 50ml or 100ml with 0.9 per cent w/v saline. Power/time data were recorded at 25C, 30C, 37C and 45C using isothermal calorimetry. Degradation rate constants were determined from the slopes of ln (power) versus time graphs and an Arrhenius plot was used to extrapolate the stability data to 6C (the storage temperature of the reconstituted solution). RESULTS — Busulfan degradation was found to be first-order at all study temperatures and linear Arrhenius plots were obtained. Extrapolation of the data to 6C gave predicted rate constants of 3.9 x 10 –7 s –1 and 9.7 x 10 –7 s –1 for dilution to 50ml and 100ml, respectively.