Polysaccharide quasi-solids containing a large excess of water were proposed as a new medium for photochemical reactions. Photochemical reaction was studied by the electron-transfer quenching of photoexcited tris(2,2'-bipyridine)ruthenium(II) (Ru(bpy)(3)(2+)) by methyl viologen (MV2+) or propyl viologen sulfonate (PVS0). The maximum wavelengths of the visible absorption and emission spectra of Ru(bpy)(3)(2+) were 452 and 627 nm, respectively, in both agarose and kappa-carrageenan quasi-solids, similar to those in an aqueous solution. Photoinduced electron transfer from the photoexcited Ru(bpy)(3)(2+) to MV2+ took place by a dynamic mechanism in a 2 wt % agarose/water quasi-solid, the same as in pure water, but occurred by both the dynamic and static mechanisms in a 2 wt % kappa-carrageenan/water quasi-solid in high MV2+ concentration regions ([MV2+] > 1 mM). Since kappa-carrageenan contains anionic sulfonate groups, the cationic Ru2+ complex and MV2+ were supposed to be bound electrostatically near the main chain, so that the static electron transfer would take place. On the contrary, photoinduced electron transfer from the Ru(bpy)(3)(2+) to PVS0 took place by a dynamic mechanism even in the kappa-carrageenan quasi-solid because of the neutral acceptor. The electron transfer in the kappa-carrageenan was analyzed with a mechanism involving both the dynamic and static electron transfer by assuming a multistep incorporation of the MV2+ into the static quenching sphere around the Ru complex. The electron-transfer rate constants from the photoexcited Ru(bpy)(3)(2+) to MV2+ were 4.7 x 10(8) M-1 s(-1) in the agarose quasi-solid and 2.1 x 10(9) M-1 s(-1) for the dynamic part in the k-carrageenan quasi-solid. The latter value is approximately 1 order of magnitude as large as that in a homogeneous aqueous solution. Thus, the tight and elastic quasi-solid can in principle work as a medium like liquid water. However, the nanoscale heterostructured main chain can provide a kind of molecular surface where interaction of the reactants controls the mechanism and kinetics, as shown in the kappa-carrageenan case.