The geometry of molecules in ionized states differs from that in their ground states. Various mechanisms which could be responsible for changes in bond length on ionization are discussed, and calculations are presented for tetrachloroethylene. Three mechanisms are considered : (i) change in bond order, (ii) change in electrostatic interactions, and (iii) change in repulsive exchange forces. For the particular example discussed, mechanism (i) accounts satisfactorily for the experimental results. There is not sufficient accuracy in the molecular wave functions, nor sufficient comparative experimental observation, to estimate the importance of mechanism (ii). Mechanism (iii) can be ruled out for practical calculation, since presently available wavefunctions are not good enough.