It has been predicted by Pons and Dantzer that temperature gradients due to the released enthalpy of H absorption-desorption will generate macroscopic inhomogeneities of the αβ phase proportions in metal hydrides. We used in situ X-ray diffraction and in situ neutron diffraction respectively to study the growth of β-LaNi5H at the free surface, and β-LaNi5-D in the bulk of powdered samples. It was found that a macroscopic compositional inhomogeneity does occur, and can be so severe that the free surface of the sample remains pure α phase while the bulk of the sample is rich in β phase.