Gelation is enhanced by the addition of sugars and polyols. How, at a microscopic level, do such cosolvents enhance gelation? The following two different hypotheses have been proposed so far to answer this question: (i) enhancement of water structure around the biopolymer induced by cosolvents; (ii) exclusion of cosolvents from biopolymer surfaces. To examine the validity of the above hypotheses, as well as to quantify the driving forces of cosolvent-induced gelation, we have constructed a statistical thermodynamic theory of gelation, by extending our KirkwoodBuff theory of cosolvency; biopolymerwater and biopolymercosolvent interactions can both be determined from thermodynamic data. The exclusion of cosolvents is shown to be the dominant contribution, whereas the hydration change is a minor contribution, which may be important only so far as to mediate the exclusion of cosolvents.