Quantitative on-line NMR spectroscopy is used to study the kinetics of the reaction of aqueous formaldehyde and urea. The investigation focuses on the formation of low molecular mass compounds during the methylolation step. The experiments were carried out at overall formaldehyde to urea molar ratios between 1:1 and 4:1, pH values between 6 and 8, and temperatures between 313 and 353 K. The experimental data were used to develop a kinetic model based on the true species concentrations. The model describes the experimental data well and can be used to predict the composition of the reacting mixture of aqueous formaldehyde and urea during the methylolation step as a function of time.