Formaldehyde is an important C1 synthon used in the chemical industry. Formaldehyde is mainly used in aqueous solutions. Solid precipitation from these solutions in the form of poly(oxymethylene) glycols impedes many technical processes. Acids, ionic liquids, and salts have been widely used as catalysts and additives in many technical processes using formaldehyde solutions as reactants. Few data are available in literature on solubility of formaldehyde in aqueous solution containing acids, ionic liquids, and salts. In the present work solubility data is reported for temperatures between 58 degrees C and 87 degrees C to investigate the effect of typical acids (H2SO4, H3PO4, CH3SO3H, CF3SO3H, C6H5SO3H, C7H7SO3H, C8H9SO3H, C10H16SO4 or H-4[ Sick/1/30mM; the acid concentration is 0.10 mol (kg formaldehyde aqueous solution) -1), imidazolium ionic liquids ([mim][HSO4], [mim][CF3SO3], [C(3)Smim][CH3SO3] or [C(3)Smim][CF3SO3]; [mim](+) and [C(3)Smim](+) stand for 1-methylimidazolium and 1-propylsulfonic-3-methylimidazolium cations; the concentration of the ionic liquid is 0.10 mol (kg formaldehyde aqueous solution)(-1)), and salts [NaCl, KCI, LiCl, MgCl2, ZnCl2, CH3SO3Na, CF3SO3Na, C6H5SO3Na, (C2H5)(4)NBr or ZI'S (1-methylimidazolium propanesulfonate); the concentration of the former nine salts is 0.40 mol (kg formaldehyde aqueous solution)(-1), and the ZTS concentrations are 0.10 and 0.15 mol (kg formaldehyde aqueous solution)(-1), respectively] on the solubility of formaldehyde in aqueous solutions. The results indicate that most acid catalysts and additive salts considered show salting -out effect. More importantly, the acids and salts that can be used as the catalysts and additive salts for trioxane and have salting -in effect are reported. The influence of the structural characteristics of the acids, ionic liquids and salts that were added on their salting-in/out effect is briefly discussed. (C) 2017 Published by Elsevier B.V.