A process is described for preparing a solution formed by at least one cobalt and/or nickel salt of at least one heteropolyanion combining molybdenum and cobalt or molybdenum and nickel in its structure, said process comprising: a) mixing at least one source of molybdenum and at least one oxidizing compound in aqueous solution to synthesize peroxomolybdate ions at an acidic pH, the (oxidizing compound/molybdenum source) mole ratio being in the range 0.1 to 20; b) introducing at least one cobalt precursor and/or at least one nickel precursor into the solution from step a) to form a solution comprising at least said salt in which the (Co+Ni)/Mo mole ratio is in the range 0.25 to 0.85.