Structural properties of phosphoglucose isomerases isolated from a variety of species have been compared by peptide fingerprinting, predicted amino acid sequence homologies and by denaturation and renaturation studies. The enzymes are more readily denatured in guanidinium chloride than in urea, and the isomerase isolated from yeast is more stable toward acid pH than the rabbit muscle enzyme. The rates of guanidinium chloride-induced denaturation are markedly increased by ionic strength and decreased by substrates, competitive inhibitors or glycerol. The enzyme can be renatured, but only in the presence of glycerol. The renaturation process is dependent on protein concentration and temperature and provides a method for the formation of mixed species heterodimers.