Four Cu-ZSM-5 samples with different average crystal sizes (from about 20 nm to 10 mu m) were synthesized and tested in the catalytic decomposition of N2O (N2O = N-2 + 1/2O(2)). Against our intuition, we found that Cu-ZSM-5 with the smallest crystal size (about 20 nm) showed the lowest catalytic activity whereas Cu-ZSM-5 with the biggest crystal size (about 10 mu m) showed the highest catalytic activity, although their Cu contents were similar. Relevant characterization was conducted employing XRD, SEM, TEM, ICP, XRF, N-2 adsorption desorption, CO-IR, H-2-TPR, and O-2-TPD. The influence of co-fed O-2 or H2O on catalytic activity was also studied. @ 2015 Elsevier B.V. All rights reserved.