It is well-known, that humidity and thus water absorption have a strong influence on electrical and optical properties of electrochemical devices containing layers of PEDOT:PSS. Sorption isotherms have to be determined to correlate the air humidity with the exact amount of absorbed water. Here we report on phase equilibrium data for different commercially relevant types of PEDOT:PSS. The measured data can be correlated well by a Flory-Huggins approach with composition-dependent interaction parameter. (C) 2010 Elsevier B.V. All rights reserved.