The muon hyperfine coupling constant (A(mu)) of the muoniated cyclohexadienyl radical (C(6)H(6)Mu) has been directly measured in a S mM solution of benzene in water by the radio-frequency muon spin resonance (RF-mu SR) technique. The relative shift of A(mu) in aqueous solution compared with the value in neat benzene (Delta A(mu)/A(mu) = +0.98(5)% at 293 K) can now be compared directly with theoretical predictions. Application of the RF-mu SR method to other dilute systems will provide extremely important information on understanding solvent effects.