We demonstrate control of gelation kinetics in formation of poly(ethylene glycol) (PEG) hydrogels by enzymatic cross-linking. A predictive model for gelation kinetics based on macromer structure and composition, stoichiometric ratios of reactants, cross-linking enzyme concentration, and the underlying kinetics of enzyme reaction was developed on the basis of classical Flory-Stockmayer theory. Experiments with substrate-functionalized multiarm comb PEG showed good agreement with theory upon variation of the enzyme concentration, with a slight overprediction in the time to gelation. Experiments with a substrate-functionalized difunctional PEG in conjunction with a polypeptide where macromer concentrations were varied were also consistent with theory, but with a slight underprediction of gelation time.