The stability of fumonisins B1 and B2 (FB1 and FB2) during 6 week storage was studied in acetonitrile:water (1:1) and methanol at four storage temperatures (-18, 4, 25 and 40 degrees C). Samples were analysed by HPLC with fluorescence detection after derivatization with o-phthaldialdehyde. Decomposition of fumonisins was evident in a methanol solution after 6 week storage at 4, 25 and 40 degrees C (signals for both toxins decreased by 5, 35 and 60%, respectively), whereas no losses were observed in samples stored at -18 degrees C. Both fumonisins were quite stable in acetonitrile:water solution at any of the four storage temperatures. The stability of fumonisins in acetonitrile:water (1:1) was confirmed over a period of 6 months at three temperatures (-18, 4 and 25 degrees C) in two solutions at different mass concentrations. No significant difference (t0.95-test) was observed between solutions stored in the dark at different temperatures and for different times (3 and 6 months). The overall coefficient of variation (n = 24, four measurements at each storage time and temperature) in each solution was < 3% for both toxins. Acetonitrile:water (1:1), unlike methanol, is a suitable solvent for fumonisin reference solutions.