[en] The presence of Cu(II) in the isotopic exchange reaction for the labelling of N-isopropyl-para-iodo-amphetamine with ¹²³I is proven to cause radioiodinated as well as cold side-products. The use of Cu(I) for promoting the nucleophilic isotopic exchange in the presence of an excess of reductant, such as Sn(II) or ascorbic acid, allows an almost quantitative yield to be obtained within 20 minutes. The reaction is assumed to proceed through an organo-copper complex favouring the exchange of the radioiodide. The labelling procedure is suitable to be adapted as a kit-form preparation. (author)