Ion composition contained in individual rain droplets provides important information to investigate the chemistry in rain and clouds, but general rain sampling equipment temporally and spatially averages the information. Determination of the SO concentration in an individual rain droplet was achieved by precipitate production in synthesized acrylamide polymer gel. Concentration of the target ion was calculated from the droplet print diameter and precipitation area measured from digital microscope images. We investigated the effects of the interior electrolyte concentration in the polyacrylamide gel and UV irradiation time on the physical properties of the gel and precipitate formation. The precipitated components were identified by scanning electron microscopy with energy dispersive X-ray analysis. We also clarified the effects of coexisting ions on the reaction between the interior and exterior electrolytes. For actual rainwater, the SO concentration estimated by this method was in agreement with the results obtained by ion chromatography. 4 4 2- 2-