Poly(ethylene glycol) (PEG) hydrogels are water-swellable, non-toxic, non-immunogenic, and biocompatible. In this paper, we describe the generation of biodegradable PEG hydrogels by cross-linking biotinylated PEG oligomers containing intrinsic disulfide bonds via biotin-avidin interactions. The biotinylated PEG oligomers were synthesized by the condensation reaction between PEG and 3,3′-dithiodipropionic acid, followed by the reaction with biotin. This methodology obviates the need for potentially toxic chemical cross-linking agents that are usually used in the common preparation of hydrogels. Therefore it may be particularly useful in biomedical or pharmaceutical applications.