We herein present a quantum chemical method for predicting the thermodynamic properties of a reactive formaldehyde + water system. The TURBOMOLE program was used to obtain the sigma profiles of poly(oxymethylene) glycol oligomers produced via a series of chemical reactions in aqueous solution. The conductor-like screening model for real solutions (COSMO-RS) method, in combination with chemical equilibrium constants and the mass balance equation obtained using MATLAB, was successfully used to calculate the activity coefficients of all compounds in the liquid. Based on NMR data from the literature along with the obtained activity coefficients, we modified the thermodynamic chemical equilibrium constant K-n* for glycol formation. Finally, we estimated the vapor-liquid equilibrium and distribution of oligomers (MG(n), n >= 1) in the formaldehyde-water solution. The COSMO-RS method was thus proven to be an efficient tool to study formaldehyde-containing aqueous solutions, and can be used as a basis for other similar reactive systems. (C) 2016 Elsevier B.V. All rights reserved.