Microsphere systems have been used to deliver drugs, proteins, and cells. However, dispersed microspheres can induce harmful effects after they are introduced into the body. To agglomerate these dispersed microspheres, an in situ forming system was developed to fabricate microsphere/hydrogel composites. Polycaprolactone microspheres with a porous surface and hollow core were physically incorporated into a genipin-crosslinked gelatin hydrogel. The incorporation of microspheres reduced the swelling capacities and weakened the unexpected volume expansion, which is not favorable for in vivo implants. Additionally, the reacted genipin ratio increased with increasing gelatin concentration. The degradation of the composite was also determined, and it was proposed that the degradation mechanism of the composite was bulk collapse, whereas that for the pure hydrogel was surface erosion. The obtained microsphere/hydrogel composite might have a great potential application as an injectable system for tissue regeneration. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013