Soybean, cowpea and groundbean, three locally grown legumes in West Africa, were processed into tempe, an Indonesian-type fermented food. Changes in oligosaccharides, trypsin inhibitor, phytic acid and tannins were monitored during the pretreatments (soaking and soaking-dehulling-washing-cooking) and fermentation with Rhizopus oligosporus. About 50% of raffinose and more than 55-60% of sucrose and stachyose were lost during the pretreatments of the beans. Stachyose decreased during fermentation with a reduction of 83.9%, 91.5% and 85.5% respectively for soybean, cowpea and groundbean while raffinose remained fairly constant. Galactose, the predominant sugar, glucose, fructose, maltose and melibiose increased during the first 30 and 36 h of fermentation of cowpea and groundbean, but decreased thereafter. Soaking the beans for 12-14 h had no effect on the level of trypsin inhibitor of the beans while it increased the phytic acid content to 1.7% in soybean and to 0.8% and 0.7% in groundbean and cowpea. Cooking soaked and dehulled soybean, cowpea and groundbean for 30, 7 and 15 min respectively resulted in 82.2%, 86.6% and 76.2% in trypsin inhibitor. However, a slight increase in trypsin inhibitor was observed during soybean fermentation. Phytic acid decreased during fermentation by 30.7%, 32.6% and 29.1% respectively in soybean, cowpea and groundbean at the harvesting time. Tannins mainly located in seed coat were removed as a result of pretreatments mainly dehulling. These changes are beneficial especially in infant feeding based on cereal and legume-based foods. (C) 2002 Elsevier Science Ltd. All rights reserved.