Urea-formaldehyde (UF) resins are prone to hydrolytic degradation, which limits their use to indoor applications. This study examined the modification of UF resin with various thermoplastics as a means to increase the moisture resistance of the adhesive. UF adhesives were modified in situ with various hydrophobic and hydrophilic thermoplastic formulations, using either polar or nonpolar initiators. Unmodified and modified UF resins were characterized in terms of viscosity, pH, and gel time in their prepolymer suspension state. Cured solid UF resin plaques were prepared to isolate moisture sorption effects of the cured UF resin from that of the wood component in composites, which dominates their moisture uptake. Relative crosslink density and moisture sorption tests were run on cured UF resin plaques. Results indicated that viscosity increased after modification in most cases, with higher viscosities resulting from formulations using an acidic (polar) initiator. In all cases, activation energies of the curing reactions of thermoplastic-modified UF suspensions were lower than the unmodified UR High relative crosslink density compared to the unmodified UF was found for one sample, which correlated well with lower overall moisture sorption. Higher relative crosslink density of cured UF resin plaques appeared to be an indicator of lower moisture uptake. (c) 2006 Wiley Periodicals, Inc.