Catalysts for ammonia synthesis based on ruthenium, directly supported over graphite having BET specific surface area in excess of 10 m.sup.2/g, preferably in excess of 100 m.sup.2/g and more preferably in excess of 280 m.sup.2/g, the graphite being characterised by X-ray diffraction pattern containing the diffraction lines characteristic of the crystalline graphite only, with exclusion of relevant bands due to amorphous carbon, to which catalysts barium, caesium and potassium are added as promoters. The graphitic supports allow to avoid the known pre-treatments and post-treatments needed in the case of supports obtained by partial graphitisation of active carbons and during use they are affected negligibly by the methanation shown by the supports obtained from active carbons. Furthermore, the catalysts of the present invention are characterised by a very high activity, even with ruthenium loadings well below the known loading.