[en] Highlights: • We constructed and applied a lightweight portable dissolved gas sampling device. • A membrane contactor has been used to sample the dissolved gases from the water. • Gas compound and gamma spectrometric measurements were done from the samples. - Abstract: This article describes a novel sampling method for dissolved gases from radioactive waters. The major aim was to build a portable, lightweight sampling device in which the gas sample container is not in contact with the water itself. Therefore, a membrane contactor was used to take representative dissolved gas samples from the water of spent fuel pools. Quadrupole mass spectrometric and gamma spectrometric measurements were made from the samples to determine the gas composition and to detect any radioactive gas of fission origin. The paper describes (i) the construction of the sampler in general, (ii) the operation of the sampling unit and (iii) the measurement results of the first samples and the interpretation of the data. Both small and large fluctuations were able to be detected when the freshly spent fuel rods were put into the spent fuel pool or when the head valves of the toques of the fuel rods were replaced. In the investigated period (2013–2014), the main gas composition did not show large fluctuations, it was close to the composition of dissolved air. However, the activity concentration of "8"5Kr varied in a broad range (0.001–100 kBq/l).