Phosphinousacid or phosphane oxide? Both tautomers of (C2F5)2POH (see picture) are found in the neat liquid, whereas only phosphinousacid is present in the gas phase or in solution. The synthesis starting from (C2F5)3PF2, the thermodynamics of the tautomerization, and the detection and thermodynamics of the cis‐/trans‐POH rotamers of the acid are described.