AbstractMerbau wood has a disadvantage in outdoor applications because its water-soluble extractives readily leach out and stain adjacent materials. This study examined the thickness of the heat-treated wood specimen on water-soluble extractives and some mechanical properties. The results show that heat treatment is efficient at removing the water-soluble extractives of merbau heartwood and overcoming the leaching problem. The absorbance of UV light decreased in intensity when the duration of heating was increased from 1 hour to 5 hours. The dominant absorbance peak increased from the surface layer to the core layer. The modulus of elasticity (MOE) and modulus of rupture (MOR) of heat-treated samples showed significant reductions when the heating duration exceeded a certain point. The total loss of mass during heat treatment and water extraction verified that heat treatment removes the water-soluble extractives efficiently.