AbstractBiomass content greatly affects the properties of wood-plastic composites (WPCs). Determination of the biomass in WPCs is important for the development of WPCs. In this study, transmission Fourier transform infrared (FTIR) spectroscopy was used for biomass quantification in the following WPCs: Moso bamboo/polypropylene (PP) composites, Chinese fir/PP composites, and poplar/PP composites. The bands in the region of 1060 to 1030 cm-1 were considered characteristic of biomass. The peak at 1377 cm-1 was typical of PP. The peak intensities ratios (PIRs) of biomass to PP were determined, and the biomass contents were plotted against the PIRs. The achieved coefficients of determination (R2) of the calibration fits exceeded 0.96. The results of validation showed that the range of the relative prediction deviations for biomass within WPC species was lower than ± 5.0%. Additionally, all three WPC species were combined into one data set, and a mixed model was constructed that had a slight decrease in the quality of the correlation (R2 = 0.93). The range of the relative prediction deviations for biomass between WPC species did not exceed ± 9.0%.