Although addition of montmorillonite (MMT) clay to isotactic polypropylene (iPP) results in large increases in the melt viscosity at low shear rates, satisfactory impregnation of glass mats is obtained with iPP/MMT under conditions typical of industrial processing, and the MMT is well dispersed throughout the resulting composites. However, the MMT is also shown to retard relaxation of the glass mat, leading to non-uniform through-thickness glass fiber distributions. Thus, depending on the initial lay-up and hence on the initial surface fiber concentration, the bending modulus of the consolidated specimens may either increase or decrease with increasing matrix MMT content, whereas the tensile modulus remains consistent with the predictions of micromechanical models based on the assumption of a uniform glass fiber distribution. The presence of matrix-rich layers at surfaces of specimens containing MMT is also shown to promote crack initiation and induce premature failure in flexural tests.