Based on the extensive experiments of Qin et al.(9) on the effects of low molecular weight liquid-rubber diluents on the craze plasticity of polystyrene and the substantial toughening obtainable in certain ranges of temperature, strain rate, and diluent character, we examine in this article the various factors that limit the beneficial toughening effect of such diluents. We find that the principal limitation is the inability of the diluent to keep up with the local plastic deformation process which it affects autocatalytically. A subsidiary limitation is the rate of wetting of the craze borders by the spreading diluent rubber as the latter is released from submicron size diluent pools by the craze itself. While we find it difficult to quantitatively separate in importance the diluent spreading effect from the actual effect of plasticization associated with the autocatalytic sorption of the diluent at the deforming craze borders, the latter appears to be the dominant effect which eventually limits the toughening.