The effect of the possible cell defects in the methane-hydrate crystal on the chemical shift of the carbon core has been investigated by DFT calculations. The clathrate structure was modeled as a water monolayer, in mono and bicavital clusters. The results show that the radius of the clathrate cell correlates with the chemical shielding of the carbon core. The calculated chemical shifts for the most stable clusters are in good agreement with the observed NMR signals supporting the previous spectral assignment. The occupancy of a neighbouring cell in a bicavital cluster has a small effect on the calculated NMR shift. (C) 2010 Elsevier B.V. All rights reserved.