The present invention provides a method for the synthesis of radioiodinated compounds which is advantageous over prior art methods. Using a hydrazine or an aminoxy in place of a primary amine for indirect radioiodination facilitates a much quicker reaction thus reducing reaction time and increasing the yield. In addition, where there are primary amines in the molecule to be radioiodinated, such as the N-terminus of a peptide or lysine residues, reaction at the hydrazine or aminoxy is greatly favoured.