An active catalyst in the preparation of alternating copolymers of olefins with carbon monoxide (CO) and consisting of:a) a metal salt of the group IB of the periodic table;b) a bidentate binder containing two nitrogen or phosphorous atoms capable of binding themselves to the metal atom by means of co-ordination bonds;c) a Lewis acid;d) optionally an oxidizing agent.The preparation of the catalyst is described together with its use in the alternating copolymerization of ethylene (C2) and/or other olefins with carbon monoxide.