NOVELTY - The production of an electrocatalyst or electrocatalyst ink comprises: (A) mixing at least an ionomer with at least a support or at least a precursor compound of a catalytic active substance; (B) mixing the obtained mixture with the precursor compound of a catalytically active substance, when the support material was added; and (C) precipitating the catalytically active substance by chemical reduction of the precursor compound, where the reduction is carried out in combination with step (B), when ionomer and support material had been mixed in step (A).    USE - The electrocatalyst or electrocatalyst ink is used for the production of a catalyst coated membrane, fuel cell and gas diffusion electrode (claimed).    ADVANTAGE - The process provides electrocatalysts or electrocatalyst ink without using high expensive apparatus and explosive solvent, and is therefore economical.    DETAILED DESCRIPTION - The production of an electrocatalyst or electrocatalyst ink comprises:    (A) mixing at least an ionomer with at least a support or at least a precursor compound of a catalytic active substance;    (B) mixing the obtained mixture either with the precursor compound of a catalytically active substance, when the support material was added, or with the support material when the precursor compound of the catalytically active substance was added; and    (C) precipitating the catalytically active substance by chemical reduction of the precursor compound, where the reduction is carried out in combination with step (B), when ionomer and support material had been mixed in step (A).    INDEPENDENT CLAIMS are also included for the following:    (1) an electrocatalyst and electrocatalyst ink, obtained by the above process;    (2) an electrocatalyst comprising the support material, ionomer and the catalytic active substance, where at least 70% of the surface of the electrocatalyst contains catalytically active substance, and between the catalytically active substance, ionomer and support material, a 3-phase system is formed;    (3) an electrocatalyst ink comprising the electrocatalyst and at least a solvent;    (4) the production of an electrocatalyst ink comprising dissolving or dispersing the electrocatalyst;    (5) a catalyst coated membrane comprising the electrocatalyst;    (6) a fuel cell comprising the catalyst coated membrane;    (7) a gas diffusion electrode comprising the electrocatalyst; and    (8) a membrane-electrode unit comprising the electrocatalyst, catalyst coated membrane or gas diffusion electrode.