Muscle relaxant formulations which include one or more quaternary ammonium neuromuscular blocking agents have a reduced tendency for hydrolytic degradation, and therefore a longer shelf life stability, when combined with one or more organic anions having at least six carbon atoms and having a pKa of less than 4.0 (preferably ranging from 0.5 to 3.5). Particularly good results are achieved when using acids of very low solubility in water, such as gentisic acid which is less than 1% soluble at room temperature.