We have succeeded in fabrication of lead-halide single crystalline heterostructures by physical vapor deposition. By coevaporating CH3NH3I and PbI2 onto atomically flat CH3NH3PbBr3 (001) surfaces at room temperature, pseudomorphic perovskite alloy CH3NH3Pb(Br1-xIx)(3) thin films are epitaxially formed. Formation of alloys proceeds by interdiffusion of I and Br ions via halide ion vacancies. Continuous ion exchange results in exponential decay in iodine composition x with a time constant of about one month. We suggest that large lattice mismatch between CH3NH3PbI3 and CH3NH3PbBr3 may accelerate alloying driven by the ion migration. (C) 2019 The Japan Society of Applied Physics