Resonance Raman (RR) spectra of Mg-Etioporphyrin I (Mg-Etio I) and its stable isotopically substituted derivatives /(14/15)N (24/26)Mg-/ in solution in 1,2-dichloroethane and in solid state in KIBr have been obtained. New spectroscopic information (isotope effect; triple correlation between vibrational RR shifts, loss of JL* conjugation and porphyrin core contraction; anomalous polarization; …) forms a basis for the discussion on the assignment of the vibration modes and the determination of the structure of metalloporphyrins and their analogues.