This protocol describes a detailed procedure for the synthesis of N-succinimidyl 4-guanidinomethyl-3-[*I]iodobenzoate ([*I]SGMIB), an agent useful in the radio-iodination of proteins, including monoclonal Abs, and peptides that undergo internalization after receptor or antigen binding. In this procedure, the tin precursor N-succinimidyl 4-[N1,N2-bis(tert-butyloxycarbonyl)guanidinomethyl]-3-(trimethylstannyl)benzoate (Boc-SGMTB, 3) was first radio-iodinated to [*I]Boc-SGMIB, a derivative of [*I]SGMIB with the guanidine function protected with Boc groups. Treatment of [*I]Boc-SGMIB with trifluoroacetic acid delivered the final product. The total time for the synthesis and purification of [*I]Boc-SGMIB and its subsequent de-protection is approximately 140 min.