AbstractA convenient method is given for the semiquantitative determination of those changes ΔSiin internal coordinates, which describe the structure of the radical cationM+(Ψ) relative to the neutral moleculeMin its electronic ground state. The changes ΔSicorrespond to those associated with the relaxation process which follows the ionisationM+hv→M+(Ψ) +e,M+being in the electronic state Ψ. The method, based on the procedure originally developped bySmith&Warsop[3], uses the spacing and relative intensity of the vibrational fine‐structure components of the bands in the photoelectron spectrum ofM.From these data are calculated the changes ΔQkin normal coordinates, which are then transformed into the ΔSiby applying theL‐matrix.The changes ΔRCX, ΔRCCand ΔRCHwhich describe the structure of the radical cations of the four monohaloacetylenesI(X) (X = F, Cl, Br, I) in their2IIΩ(1) and2IIΩ(2) states have been determined. It is shown that the results are in agreement with theoretical expectation and that they confirm the band assignement proposed in a previous