There are over 100,000 patients on organ transplant waiting lists, creating a significant need to expand the donor pool. The heart is the most difficult organ to preserve ex vivo, with a short viable storage time of 4-6 hours, because damage to mitochondria during preservation can impair the heart's contractile function. By extending the viability time, the geographical range of donors can be extended. Hydrogen sulfide (H2S) has been shown to reduce metabolism, increase preservation times, and enhance graft viability. We have developed gelatin microspheres under 10 microns able to slowly release H2S and investigated different crosslinking concentrations to understand the time release profiles. These microspheres were then used to maintain H2S levels in cardiomyocyte cell cultures without decreasing cell viability. Histological samples from 20 cold-stored rat hearts in various experimental treatments show H2S-releasing microspheres offer protection against preservation injury comparable to the current clinical standard, University of Wisconsin solution.