Biocomposites were prepared by solvent casting from poly(vinyl) alcohol and mechanically microfibrillated (MFC) birch pulp. Preparations varied in concentration of polymer solution, mixing time of cellulose/PVA dispersion and were either degassed or not. In addition, specimens were tested in different levels of relative humidity at a constant temperature. The results show that the preparation method has a significant influence on mechanical properties. It was observed that aggregation of microfibrills was easier at lower concentrations of polymer solution leading to inferior properties of composites. Degassing provided increase in Young's modulus and tensile strength at MFC loads below 10% (w/w). However, there was no significant change in composites density. Longer mixing time significantly deteriorated mechanical properties of biocomposites. The relative humidity proved to be very important factor. The properties of composites conditioned at 45% RH showed higher strength and stiffness properties than conditioned at 55 %RH. The experimental data were in a good agreement with percolation model.