Satisfactory impregnation of glass fiber mats may be obtained with isotactic polypropylene/montmorillonite (MMT) nanocomposites under conditions comparable with industrial conditions. However, it is demonstrated here that the high melt viscosity of the nanocomposite matrix at low shear rates may significantly influence the release of the compressive load in the glass mat and hence the glass fiber distribution in consolidated specimens. Thus, depending on the initial lay-up and overall glass fiber content, the bending modulus may either increase or decrease with increasing MMT content, whereas the tensile modulus is more consistent with micromechanical models assuming a uniform glass fiber distribution. Results from fractographic analyses show that the presence of matrix rich layers at the specimen surfaces may also lead to premature crack initiation and failure in flexion. © 2010 Springer Science+Business Media, LLC.