A multisite lattice gas approach is used to model pressure-optical- transmission isotherms (PTIs) recorded by hydrogenography on Mgy Ti1-y Hx sputtered thin films. The model reproduces the measured PTIs well and allows us to determine the chemical short-range order parameter s. The s values are in good agreement with those determined from extended x-ray absorption fine structure measurements. Additionally, the PTI multisite modeling yields a parameter L that accounts for the local lattice deformations with respect to the average Mgy Ti1-y lattice given by Vegard's law. It is thus possible to extract two essential characteristics of a metastable alloy from hydrogenographic data.