The addition of micronized polyurethane powders obtained from waste flexible polyurethanes to ureaformaldehyde (UF) resins and to phenol-formaldehyde (PF) resins improvesmarkedly the performance of the panels prepared with these resins, namely plywood and particleboard. Infrared spectrophotometry (FT-IR) indicates that to some limited extent a reaction does appear to occur between micronized PUR waste powder and PF resin when the mix is cured under alkaline conditions. There appears to be no reaction at all instead between acid-setting UF resins and PUR powder. In both cases an active filler or extender effect appears to occur, otherwise the improvement in dry strength of UF–bonded joints cannot be explained. Even when reaction does evidently not occur, the addition of PUR powder improves markedly the water resistance of UF and PF resins. This active extender/filler effect is not due to any isocyanate group being re-generated on heating as both FT-IR and NMR confirm the absence of these groups.