AbstractA method for transformation of readily available beta‐vinyl 99% carbon‐13 enriched derivatives of protoporphyrin‐IX dimethyl ester (3‐5) into the less accessible alpha‐vinyl labelled isomers (6‐8) is described. The procedure involves thallium(III) promoted vinyl carbon rearrangement, and proceeds through 2,2‐dimethoxyethyl (e.g.10), formylmethyl (e.g.11), 2‐hydroxyethyl (e.g.12) and 2‐chloroethyl (e.g.13) porphyrins; the rearranged vinyl groups are regenerated from 2‐chloroethyl in the last step by treatment with base. No evidence of vinyl carbon scrambling in the sequence is observed, and spectroscopie data of the