Abstract: The proposed method of determining dew point involves directing the gas under investigation onto a cooled section of an optically transparent body through which a luminous flux is allowed to pass and recording fluctuations in the intensity of the luminous flux, the flow-rate of the gas thus directed onto the cooled section being reduced down to zero while its molecular diffusion is preserved. To limit the flow-rate of the gas impinging on the cooled section (1) and reduce contamination of the optically transparent element, the dew point gauge is provided with a sampling tube (10).