Substantially oxygen-free nitrogen, which has been prepared by passing nitrogen, containing small amounts (i.e. not more than 10 per cent) of oxygen, over finely divided coal at not exceeding 450 DEG C. or below the softening point of the coal if this is below 450 DEG C. (see Group III), is employed as an inert medium in the polymerization of lactams, e.g. caprolactam. The oxygen content of the nitrogen is reduced to below 0.001 per cent. The polymerization products are colourless, and filaments spun therefrom have good mechanical properties and are not subject to repeated breakage.ALSO:Free oxygen is removed from a gas, e.g. nitrogen, hydrogen, argon, and carbon monoxide, by passing the gas over a finely divided, i.e. not greater than 5 mm. grain size, coal at not exceeding 450 DEG C., but below the softening temperature where a softening coal is used and continuing the treatment until a substantially oxygen-free gas is obtained. Coal may be anthracite, bituminous coal, or lignite, of grain size 0.1-3 mm., and where the coal does not soften below 450 DEG C. the treatment may be effected at 360-380 DEG C. or where the softening temperature is 320 DEG C. the treatment temperature may be 270-290 DEG C. Alternatively, the temperature may be below 60 DEG C., e.g. 45-50 DEG C., but the activity of the coal is not maintained for so long. The gas contains not more than 10 per cent., preferably not more than 3 per cent. oxygen, and the content is reduced to less than 0.001 per cent. The coal may be preoxidized, either by using it for effecting the process at below 30 DEG C., or by leading air through. In an example (1) preheated nitrogen containing oxygen is passed through meagre coal at 300-350 DEG C. and then through silica gel. Some carbon dioxide, carbon monoxide, or water vapour may be formed during the process. Nitrogen treated by the process may be used for ammonia synthesis, for analysis and as inert atmosphere during the polymerization of lactams. Argon treated by the process is very suitable for arc welding.