We performed studies on pulse radiolysis of highly transparent and shape-stable hydrogels of hydroxypropyl cellulose (HPC) that were prepared using a radiation-crosslinking technique. Several fundamental aspects of radiation-induced chemical reactions in the hydrogels were investigated. With radiation doses less than 1 kGy, degradation of the HPC matrix was not observed. The rate constants of the HPC composing the matrix, with two water decomposition radicals [hydroxyl radical (^•OH) and hydrated electron ()] in the gels, were determined to be 4.5 × 10⁹ and 1.8 × 10⁷ M⁻¹ s⁻¹, respectively. Direct ionization of HPC in the matrix slightly increased the initial yield of , but the additionally produced amount of disappeared immediately within 200 ps, indicating fast recombination of with hole radicals on HPC or on surrounding hydration water molecules. Reactions of with nitrous oxide (N₂O) and nitromethane (CH₃NO₂) were also examined. Decay of due to scavenging by N₂O and CH₃NO₂ were both slower in hydrogels than in aqueous solutions, showing slower diffusions of the reactants in the gel matrix. The degree of decrease in the decay rate was more effective for N₂O than for CH₃NO₂, revealing lower solubility of N₂O in gel than in water. It is known that in viscous solvents, such as ethylene glycol, CH₃NO₂ exhibits a transient effect, which is a fast reaction over the contact distance of reactants and occurs without diffusions of reactants. However, such an effect was not observed in the hydrogel used in the current study. In addition, the initial yield of , which is affected by the amount of the scavenged precursor of , in hydrogel containing N₂O was slightly higher than that in water containing N₂O, and the same tendency was found for CH₃NO₂.