AbstractThis study was carried out to determine the thickness swelling of three-layered hybrid particleboard composed of sawdust and Acacia mangium under two different testing conditions. The experimental particleboards, composed of mixed sawdust and Acacia mangium, were fabricated with different resin contents and densities within the face and back (both composed of sawdust) and the core (composed of Acacia mangium particles). Particleboards consisting of only Acacia mangium particles were used as the control. Three different resin ratios (8:10:8, 10:10:10, and 12:10:12) were tested in combination with three different board densities (500, 600, and 700 kg/m3). Urea formaldehyde (UF), with the addition of wax, was used as a binder. The thickness swelling was evaluated using two tests: water immersion, and change in relative humidity, specifically between 10 and 90%, in accordance with the ASTM D 3502-76 (ASTM 1999) standard. The results indicated that there were significant interactions between the resin contents and the densities, which had an impact on the percentage of thickness swelling of the mixed sawdust-Acacia mangium composites in both test conditions.