We describe a thermal conductivity cell for the measurements of the composition of binary gas mixtures. The main advantage of the cell is that its response is pressure independent in the range 20–100 Torr. The apparatus, of the parallel plate type, uses a very thin printed circuit film as the hot plate in order to achieve a fast response time. The cell was designed and operated to determine the ortho‐para‐composition of hydrogen gas samples but it also works reliably as an analyser of3He–4He gas mixtures. In both cases the precision of the instrument is better than 10−2for the mole fraction.