Pine needles and eucalyptus wood particles were treated with 4, 4 diphenyl methane diisocyanate. The efficacy of treatment was assessed in terms of lowering in the contact angle, the assignment of new absorption peaks and the penetration inside the surfaces. Results indicate that needles were less wettable than the wood particles and also heterogeneitically retained isocyanate on their surfaces. A positive effect in the properties of composites was observed when pine needles were used with the composite adhesive system: monomeric isocyanate for pretreatment and isocyanate prepolymer for particle-particle bonding. Pine needles and wood particles in 50:50 ratio in the panels were optimized. It was found that composites containing both pine needles and wood particles exhibited superior mechanical properties to the composites made solely with the pine needles. During hydrothermal aging, the pine needle composites retained more internal bond strength than those made with the blended fibers. Improved resistance against natural decay was noticed with the sodium pentachlorophenate treatment and exhibiting <10% weight loss in the composites. Toxicity index based on gases emitted during the burning of composites was found in the range of 2.2-3.7 which is considered to be safe in comparison with the other similar materials. POLYM. ENG. SCI., 2013. (c) 2012 Society of Plastics Engineers