[en] Natural polymers and their derivatives were regarded as radiation degradation type polymer for a long time. However, since the successful crosslinking of CMC in so call ^paste-like status^, quite a few natural polymer derivatives were found to be capable of crosslinking via irradiation. Hydroxypropyl methylcellulose phthalate (HPMCP) is water-insoluble cellulose ether with phthalate substitution and is widely used in the pharmaceutical industry due to its resistance to hydrolysis. It was found to be radiation-crosslinkable in basic solution and some organic solvents, but the gelation dose (Dg) was rather high (ca. 50 kGy). In this study, effects of two crosslinkers, i.e. methyl N, N-bis-acrylamide (bis) and ethyleneglycol dimethacrylate (EGDMA), on the accelerating of radiation crosslinking of HPMCP was studied. Intrinsic viscosities of HPMCP solution with various concentrations (10-20%) irradiated at different dose were determined to find the critical concentration for crosslinking of HPMCP and to understand the behavior of HPMCP solution before gel formation. Ion-specific swelling behavior of HPMCP hydrogel in various kinds of salt solutions was investigated to elucidate the interactions between different ion species and HPMCP. The results showed that (1) bis was apparently effective for accelerating the crosslinking of HPMCP; intrinsic viscosities of 20% HPMCP was found to be increased with increasing absorbed dose and vice versa for 10% HPMCP, implying that in paste-like status HPMCP followed crosslinking mechanism even before the formation of hydrogel; ion-specific swelling of HPMCP hydrogel without ion-exchange treatment was in good accordance with normal polyelectrolyte hydrogel except in high concentration KF solution. These results will be valuable for the processing and designing of HPMCP-based controlled release system