Although neither V nor Ti is supposed to mix with Mg, in thin films some alloy formation is observed. On hydrogenation, these alloys show clearly different behaviors. Films of Mg V disproportionate during repeated absorption and desorption of hydrogen, in contrast to Mg Ti thin films which are structurally reversible upon several hydrogenation cycles. Using Hydrogenography on co-sputtered Mg V thin films, we observe the progressive segregation of Mg and V atoms induced by the formation of the respective hydrides. A simple multi-site lattice-gas model with long range H H interaction reproduces well the optically measured isotherms and allows to determine the evolution of the chemical short-range order parameter s with cycling. The complex features observed in the isotherms are related to microstructural changes in the film. This paper shows how hydrogenography, being an optical technique, allows us to determine thermodynamic and structural/compositional information for a very large number of metal hydrides simultaneously. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.