The postseismic lithospheric deformation is usually explained as viscoelastic relaxation of the coseismic stresses. In general, for computing the postseismic deformation, the shear modulus (μ) is relaxed, keeping either the bulk modulus (k) or the La’me parameter (A) fixed. It is shown that the two assumptions yield significantly different results. The assumptionk = const. implies that the medium behaves like an elastic body for dilatational changes which can be justified on physical grounds, but such a justification cannot be given in the case of the assumption λ = const.