The effect of local density enhancement on the diffusion-controlled reactions in supercritical (SC) fluids has been clearly detected in the recombination reaction of a vinyl cation, 1-(4-methoxyphenyl)-2,2-diphenyl cation, with Br- in SC CF3H. With increasing density of SC CF3H, the observed recombination rate as a whole decreased rapidly in proportion to 1/etaepsilon, where eta and epsilon are the bulk viscosity and dielectric constant of the medium, respectively. Near the critical density, the rate constant was found to be much smaller than the predicted value from the 1/etaepsilon relation based on the Smolchowski-Debye scheme. This deviation indicates that the local density enhancement retards the recombination rate mainly due to the enhancement of dielectric constant.