New processes for synthesis of urea-formaldehyde (UF) and melamine-fortified urea-formaldehyde (mUF) resins have been developed in the last years, motivated by the current concerns about the effects of formaldehyde on human health. All these formulations are quite susceptible to possible operation error, which can significantly influence the characteristics of the final product. The main objective of this work was to implement chemometric techniques for off-line monitoring of the product's formaldehyde/urea (F/U) molar ratio using near infrared (NIR) spectroscopy. This allows the timely implementation of the necessary corrections in case the product is off-specification. Calibration models for F/U molar ratio were developed taking into account the most relevant spectral regions for these resins, individually or in combination (7502-6098 cm(-1) and 5000-4246 cm(-1)) and using different preprocessing methods. When the appropriate spectral range and preprocessing methods are selected, it is possible to obtain calibration models with high correlation values for these resins. The best preprocessing methods were identified for three cases: UF resin (produced by strongly-acid process), mUF resin (alkaline-acid process), and a combined model that involves both UF and mUF resins. It was concluded that significantly better accuracy is obtained when a new model is developed for each particular resin system. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012