The impact of elastic and microstructural constraints on structural phase transitions is investigated by using (10-300) nm Pd-H films of different microstructures. Hydrogen induced stress mainly arises from the film's adhesion to a substrate. Stress changes the hydrogens' chemical potential mu(H), modifying the hydride phase stability. Microstructural constraints channel stress release in films. A thermodynamic model is proposed to deduce the H-H interaction energy E-HH and an effective critical temperature T-c(eff) of hydride formation in films. It allows for occasionally observed sloped plateaus of mu(H) below T-c(eff). EHH (between 15 and 30 kJ/mol(H)) and T-c(eff) (340 K to 490 K) are reduced by up to 50% compared to bulk (E-HH = 36.8 kJ/mol(H), T-c = 563 K), for all films. Concentration-dependent contributions of substrate-induced stress (of about (2-5) kJ/mol(H)) and microstructure (of about (5-8) kJ/mol(H)) are separated. For all films phase separation is still found at 300 K. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.