Streams of coke-oven gas (I) and a sweep gas (II) of high specific gravity (producer gas), heated above their dew points, are passed concurrent or countercurrent along opposite sides of a porous boundary (with pore size 1-100 times greater than the mean free path of the (I) molecules), maintaining a suitably higher pressure on the (II) side to secure a free flow from (II) to (I) and a counterdiffusion from (I) to (II), whereby the higher diffusion velocity of the light constituents (H₂) will cause a net increase in the H₂ content of (II). The specific gravity of (I) is increased by 42-50% and that of (II) decreased by 12-20%. Examples show that the molecular ratio CO:H₂ of producer gas may be widely varied permitting the production of synthesis gas.