We analyzed the sorption cycling behavior of LaNi5 and LaNi4.73Sn0.27 alloys in H-2 containing 10 and 100 ppm of CO. The effect of temperature was studied for the Sn-containing alloy. When cycling in the presence of CO, we found the reaction was strongly retarded due to surface contamination but no loss of capacity was observed when samples were given enough time for both absorption and desorption. The retardation was stronger at lower temperatures and higher CO concentration. The results also indicate that a fraction, of the adsorbed CO is released during the desorption process. For the Sn-containing alloy, a stationary state is met after about 10 cycles, with no further degradation occurring past this point. The retarding factor at 40 degrees C and 100 ppm in this condition, with respect to the kinetics in pure hydrogen, is of about 600. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.