A process for preparing a hydrated form of perindopril L-arginine, characterized in that perindopril or a perindopril salt is used as a starting material and in that the process comprises: - forming a crystallisation solution comprising perindopril L-arginine, wherein forming the crystallisation solution comprises at least: - forming a perindopril solution, wherein ethanol is used as a solvent and ethyl acetate is used as an antisolvent, and wherein the perindopril solution formed contains less than 0.5 mol. equiv. of water with respect to perindopril, - adding L-arginine to the perindopril solution, to form a crystallisation solution comprising perindopril L-arginine, - measuring the amount of water in the obtained crystallisation solution comprising perindopril L-arginine, - adjusting the amount of water in the obtained crystallisation solution by adding such an amount of water to the crystallisation solution that the amount of water in the crystallisation solution is adjusted to 1.0-1.4 molar equivalents of water, preferably 1.1-1.3 molar equivalents of water, calculated with respect to perindopril L-arginine, - adding a seeding material to the obtained crystallisation solution, wherein the seeding material is the hydrated form of perindopril L-arginine, - forming a suspension of the hydrated form of perindopril L-arginine, having a relative mother liquor flow rate in the range of 5500-6500 kg/h/m 2, - filtering the obtained suspension of the hydrated form of perindopril L-arginine, to obtain a final product, a wet filter cake of the hydrated form of perindopril L-arginine, having a dry residue value after drying in the range of 50-70%. - forming a crystallisation solution comprising perindopril L-arginine, wherein forming the crystallisation solution comprises at least: - forming a perindopril solution, wherein ethanol is used as a solvent and ethyl acetate is used as an antisolvent, and wherein the perindopril solution formed contains less than 0.5 mol. equiv. of water with respect to perindopril, - adding L-arginine to the perindopril solution, to form a crystallisation solution comprising perindopril L-arginine, - forming a perindopril solution, wherein ethanol is used as a solvent and ethyl acetate is used as an antisolvent, and wherein the perindopril solution formed contains less than 0.5 mol. equiv. of water with respect to perindopril, - adding L-arginine to the perindopril solution, - measuring the amount of water in the obtained crystallisation solution comprising perindopril L-arginine, - adjusting the amount of water in the obtained crystallisation solution by adding such an amount of water to the crystallisation solution that the amount of water in the crystallisation solution is adjusted to 1.0-1.4 molar equivalents of water, preferably 1.1-1.3 molar equivalents of water, calculated with respect to perindopril L-arginine, - adding a seeding material to the obtained crystallisation solution, wherein the seeding material is the hydrated form of perindopril L-arginine, - forming a suspension of the hydrated form of perindopril L-arginine, having a relative mother liquor flow rate in the range of 5500-6500 kg/h/m 2, - filtering the obtained suspension of the hydrated form of perindopril L-arginine,