Semi-interpenetrating polymer networks (semi-IPNs) containing poly(ethylene glycol)-diacrylate (PEGdA) and modified gelatin were prepared with 2,2-dimethoxy-2-phenylacetophenone (DMPA) as a photoinitiator. The effect of (i) initiator and PEGdA concentration, and (ii) weight ratio and type of modified gelatin on the conversion of PEGdA functional end groups was monitored in situ using attenuated total reflectance-Fourier transform infrared (ATR-FTIR). Reaction induction time was dependent on DMPA concentration and increased with decreasing DMPA concentration. Relative reaction rate was strongly dependent on both DMPA and PEGdA concentrations. Gelatin weight ratio and modification did not significantly affect reaction induction time, relative reaction rate, or reaction end time. Swelling/degradation kinetics at various aqueous conditions sought to establish relationships between diacrylate conversion and the resulting semi-IPN physical properties. Semi-IPN swelling weight ratio was strongly dependent on solvent conditions and semi-IPN exposure to gamma-irradiation. Gelatin backbone modification and UV exposure time exhibited no effect on semi-IPN swelling weight ratio. In conclusion, ATR-FTIR presents a viable means of monitoring the conversion of PEGdA functional end groups within a complex mixture. UV exposure >10 s did not significantly affect the weight swelling ratio, and supports our ATR-FTIR results that network formation reached completion before 3 min of UV exposure.