The hydrogen desorption and absorption cycles of the LiNH(2)-LiH system were investigated. We have devised a method to simultaneously measure the hydrogen capacity and the amount of ammonia emitted from the system as a by-product during the hydrogen desorption. The initial hydrogen capacity of ca. 5 mass% exponentially decreased to ca. 2 mass% after 200 cycles at 573 K in which one cycle comprises the hydrogen desorption in vacuo for 3 h and the hydrogen absorption under 3 MPa of pure hydrogen for 2 h. The ammonia concentration in the desorbed hydrogen gradually increased with repeated cycling and abruptly dropped by re-mixing the sample after 100 cycles. By comparing the observed decay rate in hydrogen capacity with the mean ammonia concentration of 0.24 +/- 0.05 mol%(NH(3)/H(2)), we found that about half the decay in hydrogen capacity can be explained by the loss of the constituent nitrogen due to the ammonia emission. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.