Controlled initiation chromium dioxide synthesis

A process for the manufacture of modified ferromagentic chromium dioxide wherein a reaction mixture paste containing trivalent and hexavalent chromium oxides is hydrothermally reacted under a pressure of from 100 to 700 bar at a temperature of 200.degree.-600.degree. C. in the presence of an antimony, iron or tellurium modifier, characterized in that, the modifier is added in aqueous solution to the reaction mixture paste prior to the application of pressure and heat, said solution being prepared by adding the modifier to an aqueous CrO.sub.3 solution and heating to effect dissolution and wherein the modified ferromagnetic chronium dioxide when incorporated into a magnetic tape exhibits lower noise, better print through, narrower coercivity distribution and more uniform switching field distribution than said modified ferromagnetic chromium dioxide produced as above but where the modifier is added in solid form to the reaction mixture paste.