Liquid mixtures of formaldehyde, water, and butynediol are complex reacting multicomponent systems in which formaldehyde forms oligomers both with water and butynediol. 1H- and 13C-NMR spectra of these mixtures are elucidated. The species distribution of the oligomers is quantitatively determined by 13C-NMR spectroscopy. The measurements cover temperatures from 293 to 366 K, overall formaldehyde mass fractions from 0.10 to 0.27gg-1, and overall butynediol mass fractions from 0.05 to 0.50gg-1. A mole fraction-based and an activity-based model of the chemical equilibrium in the studied system are developed and chemical equilibrium constants are reported. (c) 2017 American Institute of Chemical Engineers AIChE J, 63: 4442-4450, 2017