The presence of methane gas will affect a soil's strength, which will alter its resistance to submarine sliding and in turn impact offshore developments. This paper describes a constitutive model for determining the undrained shear strength of fine-grained gassy soils. The model results show that although the presence of gas has a beneficial effect on the soil strength in compressive loading, the peak strength is achieved at larger deformations. In addition, the model results showed that the undrained shear strength of gassy soils increases with increasing initial consolidation stress, increasing volumetric coefficient of solubility, and increasing initial void ratio. (c) 2005 Elsevier Ltd. All rights reserved.