Coomassie Blue G-250, a quantitative color reagent used for protein analysis, reacts with tannin and polysaccharides found in cane juice and raw cane sugar. For this reason, the dye could not be used as a reagent for protein in these products. However, spectroscopic data showed that the complex formed with phenolic compounds possessed a different maximum than the protein complex. The spectrum of the complex between the dye and the nondialyzable fraction from raw sugar indicated the presence of phenolic moieties attached to this high-molecular weight fraction. The reaction of several common plant polysaccharides and tannins with the dye could constitute a serious source of error in its use to determine protein in plant extracts.