The relativistic time-dependent density functional theory has been applied to 3d photoionization along the Xe isoelectronic sequence, from I⁻ to La³⁺. It is seen that interchannel coupling between the two spin–orbit photoionization channels greatly alters their partial photoionization cross sections. Changes similar to those already observed for the Xe system are seen to affect the I⁻3d_5/2 partial cross section. For the elements following the noble gas in the isoelectronic sequence, Rydberg series of autoionization resonances associated with excitations into the closed channels 3d_3/2 → np_1/2,3/2; n'f_5/2 are seen to strongly modulate the total and partial 3d_5/2 cross sections between the two spin–orbit split thresholds. Moreover, the analysis of the transfer of oscillator strength from the continuum to the discrete part of the spectrum along the series has permitted us to follow the occurrence of the collapse of the 4f wavefunction with a 3d hole, which is seen to occur already in Cs⁺. A global picture emerges from the comparison of 3d and 4d ionization in neutral systems and Xe isoelectronic series.