Plasma membrane vesicles with H⁺‐ATPase activity were purified from 8‐day‐old oat (Avena sativa L. cv. Brighton) roots using an aqueous polymer two‐phase system. Of several detergents tested, only lysophosphatidylcholine solubilized the H⁺‐ATPase in an active form. Solubilization of the H⁺‐ATPase with lysophosphatidylcholine was possible in the absence of glycerol, but the ATPase activity decreased about 4–5 times as rapidly in the absence as in the presence of 30% (w/v) glycerol. The solubilized enzyme was further stabilized by ATP and protons. Addition of 1 mM ATP to the plasma membranes halted inactivation of the H⁺‐ATPase. Even in the absence of polyol compounds and ATP, the enzyme was stable for hours at relatively low pH with an optimum around pH 6.7 at room temperature. The curve for the stability of soluble H⁺‐ATPase as a function of pH closely resembles the pH curve for the activity of the H⁺‐ATPase. This suggests that binding of protons to transport sites may stabilize the soluble H⁺‐ATPase in an enzymatically active form.