Polypropylene powder and sisal fibers were oxygen plasma treated, and the mechanical properties of their composites were tested. Two main effects were investigated: the incorporation of oxygen polar groups in the polypropylene surface and the surface degradation and chain scission of both polypropylene and sisal fibers. Prior to these treatments, three reactor configurations were tested to invesbgate the best condition for both effects to occur in PP films. Results showed that polypropylene-cellulose adhesion forces are about an order of magnitude higher for PP f i l m treatments at 13.56 MHZ than at 40 kHz owing to much hgher chain scission at lower frequencies, although it probably also occurs at high frequency and high power. Polypropylene powder treated with oxygen plasmas in optimum conditions for polar group incorporation did not result in improvement in any composite mechanical property, probably owing to the polymer meltmg. Sisal fibers and PP powder treated in conditions of surface degradation did not improve flexural or tensile properties but resulted in higher impact resistance, comparable to the improvement obtained with the addition of compatibilizer.