The invention concerns a process for the preparation of chondroitin polysulfate with intrinsic viscosity comprised between 0.09 and 0.18 and organic sulfur content comprised between 25.8 and 37.3 % wherein chlorosulfonic acid is added to a formamide solution of chondroitin at a rate such that the temperature of the formamide solution is kept lower than 40 °C and higher than 15 °C.