The capacity of YMn2 to undergo reversible hydrogenization makes this compound a potential candidate for hydrogen storage. This report describes the successful synthesis of YMn2H6 from hexagonal C14 YMn2 as well as calorimetric studies of hexagonal C14 YMn2, cubic C15 YMn2 and their isostructural derivatives, YMn2H6 and YMn2D6. Complementary structural and physical property characterizations include X-ray diffraction, electrical resistivity, and magnetic susceptibility measurements. Of particular interest is a hysteretic first-order phase transition in C15 YMn2 near 100 K, which does not occur in C14 YMn2. Analysis of a specific heat anomaly that is associated with this transition reveals a substantial latent heat of 330 J/mol. No such anomalous specific heat exists in YMn2D6, suggesting that the transition is suppressed upon deuterization. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.