99mTc pentavalent dimercaptosuccinic acid [99mTc(V) DMSA], a useful agent for imaging thyroid medullary carcinoma and other tumors can be reliably prepared by addition of Na99mTcO4 to a freeze-dried mixture of DMSA and Sn (2:1 molar ratio). The radiochemical purity, stability and animal bio-distribution behaviour is similar to that of the agent made by addition of NaHCO3 to DMSA (III) renal imaging freeze-dried kit.