The objective of this study was to verify the effect of soaking on the factors causing flatulence in the common bean (Phaseolus vulgaris, L.) cv. IAC-Carioca during domestic preparation. A biological assay using recently weaned (21 days) male Wistar rats provided the Food Conversion Efficiency (FCE) and the Net Protein Ratio (NPR). Five treatments were carried out with isocaloric (350.9 +/- 37.9 kcal/100 g) and isoprotein (12.0 +/- 0.5%) experimental diets, with the following protein sources: beans cooked without soaking (BNS), beans soaked and cooked with the soaking water (BSWW), beans soaked and cooked without the residual soaking water (BSNW), control diet (casein) (CC), casein plus the total soluble solids found in the soaking water (CSS) for comparative purposes, and an aproteic diet (AP) for corrective purposes, all diets offered ad libitum. The contents of raffinose-type oligosaccharides were determined in the different domestic preparations of the beans. Significant reductions were observed in the contents of the oligosaccharides raffinose (25.0%), stachyose (24.8%), and verbascose (41.7%), and in the contents of total sugars (80.6%), reducing sugars (58.2%), nonreducing sugars (90.3%), and starch (26.8%) when soaking took place before cooking and elimination of the soaking water not absorbed by the beans (BSNW) was used. No significant difference (p > 0.05) was observed between the values for FCE and NPR of the control diet (casein) and control diet plus soaking water soluble solids. Neither was any significant difference between the values for the different bean treatments found, though the values for FCE and NPR were lower than those obtained for casein treatments. Thus it was verified that although the domestic preparation of the common bean significantly reduced the contents of raffinose-type oligosaccharides, total reducing and nonreducing sugars and starch, it did not interfere with its nutritive value.