Substantial pore-water tensions are set up by the removal, under undrained conditions, of the confining pressure from saturated clay samples from which the pore water has been free to drain during consolidation. These pore-water tensions control the mechanical behaviour of the unconfined samples, and in particular their strength and brittleness. In the present paper the magnitude of the pore-water tensions and the change in strength on stress release are estimated theoretically for the range of pressures within which the sample remains fully saturated. Tests on samples of two clays consolidated in the triaxial apparatus under a wide range of pressure indicate that the limiting pore-water tension above which the sample ceases to remain fully saturated is related to the equivalent pore diameter. Above this limit the loss in strength on stress release is very marked. The experimental results also show the dramatic change in brittleness resulting from high pore-water tensions. A large reduction in total stress without a change in water content is sufficient to change the failure mechanism from a plastic failure with some strain-softening to a brittle failure in which the sample shatters. The significance of the results in relation to sampling and testing of soils for engineering purposes is briefly considered.