For decades, the nitrogene hyperfine coupling constant a(N) of nitroxides has been applied to probe their environment using EPR. However, the small changes observed (approximate to 2 G from n-pentane to water) with the solvent polarity allow only a qualitative discussion. A stable beta-phosphorylated nitroxide exhibiting a small change in a(N) (approximate to 3 G from n-pentane to water) and a striking change (approximate to 25 G from n-pentane to water) in phosphorus hyperfine coupling constant a(P) with the polarity of solvent was prepared and used to develop the first procedure for the titration of water in THF by EPR, down to 0.1% v/v.