The paper presents a methodology for determining the conditions in which the liquid phase occurs in the transported natural gas. This methodology allows fast and accurate determination of the phase in which water can be found in natural gas transmission pipelines and also the possibility to highlight the points in the transmission system in which water is in liquid state. A complex function which can describe the phase behavior of water from natural gas has been defined by processing the phase envelopes of the wet natural gas in the field of the Romanian transmission system.