A new method is proposed to determine the hydrophobicity scale of porous materials exposed to gaseous water within a static or dynamical regime. The method is based on the use of 4-amino-3-penten-2-one (Fluoral-P), which can be hydrolysed by gaseous water. The kinetics of the reaction can be followed by absorption spectroscopy. An example is given with the study of the kinetics of hydrolysis of Fluoral-P trapped in various silicate xerogels exposed to humidified atmosphere, which display very different hydrophobic properties. The measurement of the half-life time of Fluoral-P trapped in the matrices reveals a very strong dependence on the exposure regime, either static or dynamical. The liquid water hydrophobicity scale, determined with the classical contact angle method and thermogravimetric analysis for the same matrices, is compared to the new gas water hydrophobicity scale. This comparison demonstrates that the Fluoral-P lifetime scale is more relevant in predicting the permeability of porous materials to gaseous water than the classical contact angle method. (C) 2010 Elsevier B.V. All rights reserved.