Polypropylene (PP) or, in some cases, poly (lactic acid) (PLA) were compounded with cellulosic fibres. The amount of fibres used was in the range 10-30 vol % and, in case of PP, a series of compounds was prepared with a minor amount of maleated PP as a compatibiliser. This matrix was denoted MAPP. Before compounding the polymers and the fibres, undelaminated bentonite (industrial scale) or delaminated clay (nanoclay) was deposited on the fibres in different amounts to improve the dispersion of the fibres in the polymer matrix, i. e., to avoid detrimental fibre bundles. The PP-based compounds were either extruded or injection moulded, whereas the PLA-compounds were only injection moulded. The mechanical properties were primarily evaluated for the injection moulded specimens. In general, the fibres had a strong effect on the mechanical behaviour of the materials, especially in the case of PLA and MAPP. Treating the fibres with undelaminated clay or nanoclay improved to some extent the dispersion of the fibres and the mechanical performance of the composites, but further optimization of the function of the mineral in this respect is probably required. The combination of the mineral treatment with a debonding agent appeared to be an interesting route here. With such a combination, a visually very good dispersion of the fibres in the PP-based matrix could be obtained, partly at the expense of the mechanical performance. The compounding of the cellulosic fibres with PP led in this case to a marked decrease in the fibre length. (C) 2007 Wiley Periodicals, Inc.