Method of preparing a radioactive rhenium complex solution

The invention relates to a method of preparing a solution of a radioactive rhenium complex, by reacting at elevated temperature a radioactive perrhenate in a substantially aqueous solution with a ligand in the presence of a reductant and optionally an antioxidant under substantially anaerobic conditions, wherein said reaction is carried out at a pH from approx. 1.5 to approx. 5 and by heating the reaction components for at least 10 min. at a temperature of at least 100° C. The invention further relates to a kit for performing said method.