Quantitative C-13 NMR spectroscopy was used to investigate the complex chemical equilibria in ternary liquid mixtures of formaldehyde-water-methanol at temperatures between 298 and 383 K. In these mixtures, formaldehyde is predominantly bound in methylene glycol, poly(oxymethylene) glycols, hemiformal, and poly(oxymethylene) hemiformals, which are formed by a series of oligomerization reactions. The present study is the first in which data on the species distribution in the studied technically important ternary system is reported. It complements previous work in which the focus was on the binary systems formaldehyde-water and formaldehyde -methanol. It also extends the temperature range in which data are available, which was previously limited by the fact that all experiments were carried out in batch cells under ambient pressure. In the present experimental study, a pressurized NMR flow cell was used for the first time. The results from the present study were obtained independently in two different laboratories with different NMR techniques and instruments. Details on the experimental procedures are presented. A comparison of the two data sets shows excellent agreement. The experimental results are compared to predictions from a recently published physicochemical. model that aims at the description of vapor-liquid equilibria in the studied mixtures. The results suggest that some model parameters should be revised if the model is to be applied to quantitatively predict liquid-phase compositions (e.g., the distribution of formaldehyde to different species) in the studied ternary system.