The present study deals with thermodynamic data for the gaseous phase of the Cs-O-H system as studied by high temperature mass spectrometry using the Knudsen cell effusion method. The vapour phase is analyzed and is composed of the monomer CsOH(g), the dimer Cs2O2H2(g), and a small amount of trimer. The vaporization behavior of CsOH(s or l) is analyzed in relation with different physico-chemical phenomena that interfere with the Knudsen method, like creeping and surface diffusion along the walls of the effusion orifice. Besides, the ionization processes are complex and render the interpretation of the mass spectrometric results difficult. Thus, calibration procedures have been carefully analyzed in order to evaluate reliably the uncertainties. The two main independent reactions that lead to thermodynamic data are the following: Cs2O2H2(g) = 2CsOH(g) with Delta(diss) H degrees(298 K) = 146.6 +/- 7.3 kJ . mol(-1) (3rd law method). CSOH(s,l) = CsOH(g) with Delta H-sub1 degrees(298 K) = 163.3 +/- 6.5 kJ . mol(-1) (3rd law method). (c) 2007 Elsevier Ltd. All rights reserved.