We explore how and by how much assumptions about elasticities of substitution affect estimates of the cost of GHG emissions reduction policies in computable general equilibrium (CGE) models using G-Cubed, an intertemporal CGE model, to carry out a sensitivity and factor decomposition analysis. The results suggest that the average abatement cost rises non-linearly as elasticities are reduced. Substitution elasticities between capital, labor, energy and materials in production have a larger impact on mitigation costs than inter-fuel substitution does. There are notable differences in the effect of the elasticities on costs at the regional level due to interactions in international trade and capital flows in such a global model. Although the results in this study are derived from a particular model, the study, in a broader sense, suggests that there is a necessity for sensitivity analysis before making any conclusive policy recommendation using CGE models.