[en] The microstructural and microchemical changes produced in a Ti endash 25Al endash 10Nb endash 3V endash 1Mo alloy (at.%) by charging at high temperatures in high pressures of hydrogen gas have been studied using TEM and x-ray methods. Hydrides incorporating all of the substitutional solutes which formed during charging have a face-centered cubic (fcc) structure and exhibit either a plate or fine-grained morphology. With increasing hydrogen content, the size of the hydrides decreases and their microchemistry changes as they approach the stable binary hydride, TiH₂. Rejection of substitutional solute elements from the hydride produces changes in the microchemistry, and consequently in the crystal structure, of the surrounding matrix. In alloys containing 50 at.% H, this solute redistribution results in the formation of an orthorhombic substitutional solid solution phase containing increased levels of Nb. The driving force for this redistribution of solutes is the reduction in the chemical potential of the system as the amount of the most stable hydride, TiH₂, forms. The hydrides reverted to a solid solution on annealing in vacuum at 1073 K, and the original microchemistry of the alloy was restored. Reversion from the hydride structure to the original α₂ ordered DO₁₉ structure proceeds via a disordered HCP phase. copyright 1996 Materials Research Society