The present invention relates to a method for complexation of a chelate with a radionuclide, advantageously gallium, the complexation being carried out advantageously at ambient temperature without heating, by adding the radionuclide to the chelate in a buffer solution, the buffer of this solution comprising between two and five functions for coordination with the radionuclide, each coordination function being independently chosen from a carboxylic acid function and a hydroxyl function, on the condition that the buffer comprises at least one carboxylic acid function and at most two carboxylic acid functions. It also relates to the injectable solution obtained.
