The antiferromagnetic spin flop transition in CuCl2·2H2O has been studied in the temperature region 1°–4.2°K by means of adiabatic measurements on a single crystal. The phase boundary obtained from isentropes in theH‐Tplane, which show a characteristic anomaly at the transition, conforms closely with that observed by NMR. Isentropic rotations of an external field in theabandacplanes of the crystal confirm a flopping of the magnetic moments in theabplane. When the applied field exceeds the critical spin‐flop field by more than 50%, however, it appears that spin flopping no longer occurs in this plane. The forms of the isentropic curves are in general agreement with those predicted from the molecular‐field model. Specific‐heat measurements in applied fields yielded the paramagnetic phase boundary up to 22 kOe, the sharpness of the specific heat singularity being maintained in the highest field applied. The Ne´el temperature atH=0 was 4.357±0.010°K with the triple point at 4.31°K, 8.50 kOe. Comparison of the sublattice magnetization vs temperature curve derived from the specific‐heat data with the predictions of the molecular field and Ising models suggests that neither of these models accurately represents the magnetic behavior of CuCl2·2H2O.