Aqueous and methanolic formaldehyde solutions are industrially important complex reacting mixtures in which formaldehyde is bound in a variety of oligomerization products. Reaction kinetics in these systems have to be taken into account in the design of reactors and separation equipment. The present paper reports on studies on reaction kinetics of aqueous and methanolic formaldehyde solutions by quantitative high-resolution on-line H-1 NMR spectroscopy. The experiments were carried out at temperatures up to 373 K partially with a pressurized NMR probe and extend the temperature range in which data were available up to now. The NMR data are used to develop a reaction kinetic model, which is consistent with a physico-chemical thermodynamic equilibrium model, for formaldehyde containing mixtures. This is an important step towards developing thermodynamically consistent process models for reactions and separations with formaldehyde containing mixtures, which take reaction kinetics explicitly into account. (c) 2004 Elsevier B.V. All rights reserved.