A thermodynamic model for liquid-liquid phase equilibrium and chemical equilibrium in the formaldehyde + water + 1-butanol system is presented. The model is an extension of the vapor-liquid phase equilibrium model for the formaldehyde + water + methanol system. Formaldehyde reacts with water and alcohols to form oligomers. New experimental results (from NMR spectroscopic investigations) are reported for the chemical equilibrium in liquid formaldehyde + 1-butanol and formaldehyde + water + 1-butanol systems at temperatures from 278 to 348 K. New liquid-liquid phase equilibrium data are reported for temperatures of 298, 313, and 333 K. The model is able to reliably describe the liquid-liquid phase equilibrium, as well as the specification in the liquid phases.