The objective of this study was to find a correlation for the viscosity changes during step polymerization of urea and formaldehyde. For this reason, a kinetic model for the condensation reactions was proposed, and using this model, a correlation for the kinematic viscosity changes of the solution during polymerization was obtained. Viscosity measurements of the samples during condensation reactions were carried out using Ubbelohde viscometers at intervals. The experimental data were curve fitted using our model, which was found to correspond very well.