An automatic Sievert's apparatus equipped with an in situ temperature-programmed desorption (TPD) analyzer was constructed to study the cyclic hydrogenation stability of the Ti25V35Cr40 alloy with minor impurities in H-2 gas. A mass spectrometer (MS) was used to examine the interactions between the impurities and the as-cycled Ti25V35Cr40 hydrides. The reactivity of the specimen with hydrogen decreased significantly. The hydrogen content of the specimen decreased severely from 0.73 H/M to 0 H/M after 82 cycles. The combined results obtained from in situ TPD-MS and the pressure-composition (P-C) isotherm curves revealed that degradation in the hydrogenation processes resulted from the formation of shield films composed of a mixture of H2S, O-2, CO, and CH4 species present in the H-2 gas source. A reactivation start-up temperature of 170 degrees C to break the shield film was found using the in situ TPD and TPD-MS techniques developed in this study. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.