A physicochemical model of the thermodynamic properties of formaldehyde-containing mixtures was revised and extended using new experimental data for vapor-liquid and chemical equilibria. The new vapor-liquid equilibrium data of the formaldehyde-water system were taken for very diluted (formaldehyde concentrations as low as 0.07 mas %) and very concentrated (up to 71 mass %) formaldehyde solutions at temperatures between 363 and 423 K and pressures up to about 650 kPa. The new chemical equilibrium data were obtained applying C-13-NMR spectroscopy to the same system at temperatures between 338 and 383 K at formaldehyde concentrations of up to about 62 mass %. Results from both investigation were used simultaneously-together with data from the literature-to improve a physicochemical model of the thermodynamic properties of aqueous solutions of formaldehyde.