Formaldehyde is one of the most important C1 building blocks used in the chemical industry. It is handled in aqueous solutions, in which it is predominantly chemically bound. Solid precipitation from these solutions in the form of poly(oxymethylene)glycols imposes restrictions on many technical processes. As previously reported literature data on the solubility of formaldehyde in aqueous solutions is old, scarce, contradictory, and only very poorly documented, new measurements were carried out in the present work. Solubility data for this reactive system is reported for temperatures between 30 degreesC and 90 degreesC, corresponding to overall formaldehyde mass fractions up to about 0.81 g.g(-1). The study covers also the influence of the addition of sulfuric acid and methanol. Furthermore, the solubility of trioxane, a stable cyclic trimer of formaldehyde in aqueous solutions, was studied at temperatures between 20 degreesC and 55 degreesC.