The major off-flavor odorants of normal (NOR) and lipoxygenase-lacking (LOX-lack) soy protein isolates (SPIs) were discriminated by sensory-directed flavor analysis. A total of 32 odor-active off-flavor compounds were detected in SPIs, and 19 of them with flavor dilution (FD) factors ranging from 3 to 2187 were quantified by external standard curves. It was found that hexanal and nonanal dominated in the off-flavor profile of SPIs regarding their odor activity values (OAVs) and FD contributions, followed by octanal, 1-hexanol, 1-octen-3-ol, 2-heptone, and benzaldehyde. To further improve the precision of quantification, the above 7 major odor-active off-flavor compounds were requantified by stable isotope dilution assays (SIDA) for the first time. The results suggested that the sensory differences between NOR and LOX-lack SPIs could be mainly due to the reduced C6/C9 aldehydes and alcohols rather than 1-octen-3-ol and benzaldehyde. Finally, these differential compounds were further verified by the spiking experiment.