The objective of this work was to study the acid-base properties of wood, poly(vinyl chloride) (PVC), nylon 6 and 6,6 by wetting and inverse gas chromatography (IGC) analyses. Information about the acid-base characteristics of these materials should be useful to improve the intermolecular bonding properties in wood-plastic composites. The acid-base properties of pine wood veneers, PVC and nylon 6,6 were determined by contact angle analysis using the work of adhesion (or Fowkes), van Oss-Chaudhury-Good (vOCG) and Chang-Qin-Chen (CQC) approaches. The IGC analysis was performed on maple wood, PVC and nylon 6 particles, and was carried out at infinite dilution using a series of both non-polar and polar acid-base probe gases. The contact angle analysis of the wood veneers using both the work of adhesion and the vOCG approaches showed that the presence of wood extractives was the dominant factor influencing the acid-base properties of the veneers. Particularly, it was shown that aging of non-extracted veneers increased and decreased their acidic and basic properties, respectively. This is presumably due to reorientation of functional groups and oxidation at the wood extractives-air interface. In the vOCG model, considerably higher base/acid ratios were obtained when using probe liquid parameters according to van Oss compared to those obtained by using liquid parameters according to Della Volpe and Siboni. Based on both the vOCG and the CQC models it was shown that nylon 6,6 had greater acid and base parameters than PVC. Additionally, the CQC model seems to be a promising tool to determine the acid-base characteristics of materials. The IGC analysis showed that nylon 6 had greater acid and base parameters than both wood and PVC which implies a strong ability to enter into acid-base interactions. The results also suggested that an increase in the basic character of wood could have the potential to improve its bonding with both PVC and nylon 6.