An advanced model of shear deformation occuring in high strain rate deformation of ductile porous solids is proposed under simple planar shear loading superposed by hydrostatic pressure. To describe porous - rigid thermoplastic material the Gurson theory of dilatant plasticity is applied and appearance of an adiabatic shear is interpreted in terms of mathematical instabilities in the underlying differential equations. The substantial influence of porosity on strain hardening and thermal softening effects is demonstrated.