Abstract: The invention relates to a process for the desulphurization of cast irons and steels.The product is constituted of the combination of magnesium shot or magnesium alloy with rounded contours having a granulometry of between 0.1 and 3 mm and a granulated slag having a basicity index higher than 1, containing alumina, lime, silica and magnesia, and optionally an additional flux having a granulometry of between 0.1 and 3 mm in a proportion ranging from 90 to 10% and preferably from 80 to 20% of magnesium shot and from 10 to 90% and preferably from 20 to 80% of granulated slag.Application to the desulphurization of forge pigs and foundry pig irons, to the nodularization of foundry pig irons by nozzle injection in a carrying gas stream into the liquid cast iron or steel.