The counting efficiency of liquid scintillators containing toluene as the solvent for a primary solute or activator and for a secondary solute or spectrum shifter is improved, particularly in the presence of water, for example, when the isotope to be detected is in aqueous solution, by adding to the liquid scintillator a hygroscopic substance which is soluble in toluene in an amount sufficient to absorb the water that is present. Such hygroscopic additive is preferably cellulosic and may be selected from the group consisting of cellulose, carboxymethylcellulose, cellulose phosphate, diethylaminoethylcellulose, benzyldiethylaminoethylcellulose, and epichlorohydrintriethanolaminecellulose.