Bitterness of soy protein hydrolysates is a major obstacle to acceptance of soy products. The objective of this sensory evaluation research was to compare the bitterness of Protex 7L-treated soy hydrolysate and 4 fractions of varying molecular weight. A fraction of low molecular weight peptides (estimated 1-5 kDa) and the unfractionated hydrolysate were identified as bitter, p = 0.009 and p = 0.088, respectively. Cohen's kappa coefficient showed fair agreement between panelist sensitivity to soy hydrolysate and leucine, but poor agreement to caffeine, quinine, and phenylalanine. Panelist sensitivity to soy hydrolysate by Fisher's exact test was independent of sensitivity to caffeine, quinine, leucine, and phenylalanine. McNemar's test indicated that bitterness in leucine and phenylalanine was different than the bitterness in soy hydrolysate. Free hydrophobic amino acids may not be responsible for the bitterness of soy hydrolysate. Small peptides may be a better standard than caffeine or quinine for bitterness training.