We experimentally investigated the changes in the ethanol concentration of an ethanol–water solution using a polarization property of the (specularly) reflected light associated with surface plasmon resonance (SPR) occurring in an aluminum diffraction grating in a conical mounting. The polarization property, which is observed as a rapid change in the normalized Stokes parameter s3 of the reflected light, can be successfully applied to refractive index sensing: the zero-crossing point of s3 precisely and easily determines the resonance angle θsp at which SPR occurs; s3 in the vicinity of θsp varies largely in response to a small change in the refractive index of a sample. Using the features of the rapid change in s3, we examined the ethanol concentration changes over a wide range from 0 to 100 percent by weight (wt%) through the measurement of θsp. Furthermore, we detected a small amount of ethanol component in water through the measurement of s3 under the angle of incidence fixed at the resonance angle of pure water.