The aim of this work was to monitor the hardening behavior of two different urea-formaldehyde (UF) resins and to investigate the possible catalytic effect of cold water extractives of different wood species on the hardening characteristic. A pure UF resin and a melamine-modified UF resin were used for this investigation. For the experiments, ammonium chloride, ammonium sulphate, acetic acid and oxalic acid at different addition levels were used as hardeners. The gel times determined at three temperatures, the pH drop of the resins after the hardener addition, the buffering behavior and changes in the complex viscosity eta* with time, as well as the development of the bond strength were chosen as parameters for characterizing the systems both with and without wood extractives. As expected, the buffering capacity of the melamine fortified resin was found to be much greater than for the pure UF resin. The gel times of the investigated resins were strongly dependent on the applied temperature, decreasing steeply at higher temperatures. Based on these data the apparent activation energies for various hardener additions have been calculated. The complex viscosity and the development of the internal bond strength indicated that cold water wood extractives had little or no effect on the curing progress of the resins.