Melamine urea formaldehyde (MUF) thermosetting wood adhesives have poor performance at elevated temperatures and humid conditions. PolyFox PF-151N polymer was mixed at different loadings (0.05, 0.1, 0.5, and 1%) with MUF to improve properties, especially water resistance and bond strength. The physical properties of the optimized MUF/PolyFox PF-151N resins were measured. In order to evaluate the quality of optimized MUF/PolyFox PF-151N resins, particleboards were produced and physical and mechanical properties were investigated. The results show that it is possible to add PolyFox PF-151N up to 0.1% to the MUF resin without altering the mechanical properties of the commercial MUF. The mechanical properties of the particleboard panels bonded with the optimal MUF/PolyFox PF-151N (99.9/0.1 by weight) resin were considerably increased as compared to the panels glued with neat MUF resin. The use of PolyFox significantly reduced 2-h and 24-h thickness swelling compared to the control panels.