The surface chemistry and dispersion in poly(lactic-acid) of microfibrillated wood and microfibrillated lignocellulose prepared from untreated and partially delignified beech were compared with conventional microfibrillated cellulose produced from bleached pulp. High heterogeneity in fibril morphology and bulk chemical composition was observed. Also surface chemistry of the fibrils was highly variable, but not clearly correlated with bulk chemistry. Composite solution-cast films of poly(lactic-acid) reinforced with 1 % fibrils were produced by adding fibrils dried from solvent into a polymer solution. Highly variable dispersion of fibrils correlated with varying mechanical performance was observed. Correlations were obtained between surface chemistry of fibrils as revealed by X-ray photoelectron spectroscopy and adhesion force microscopy on the one hand and the tensile performance of the fibril-reinforced polymer composites on the other hand. Overall, certain variants of fibrillated material with residual lignin and hemicellulose content showed reduced surface polarity, improved dispersion in poly(lactic-acid) and improved reinforcement efficiency compared to conventional MFC produced from bleached pulp.