The invention relates to a process for mixing a first acidic aqueous solution comprising hydroxylammonium and phosphate with a second acidic aqueous solution comprising nitric acid at a temperature between 20 and 80.degree. C. resulting in a third acidic aqueous solution comprising hydroxylammonium, phosphate and nitric acid, wherein in the third acidic aqueous solution the total acid concentration minus the phosphate concentration is lower than 0.523*In([hydroxylammonium]/1.25)+422/(T+81) whereby [hydroxylammonium] is the concentration of hydroxylammonium in the third acidic aqueous solution, T is the temperature of the third acidic aqueous solution expressed in .degree. C. and all concentrations are expressed in mol/l.