Using circularly polarized synchrotron radiation, the 5p pi ionization of HI molecules is studied by means of angle-resolved photoelectron spin polarization spectroscopy. The energy dependences of the dynamical photoionization parameters are obtained and compared with an ab initio calculation of Raseev et al. (1987) and the corresponding data for the xenon atom. Angular distributions of the polarization component A( Theta ) yield complete parameter sets. A strong influence of electronic autoionization is observed and analysed in terms of partial contributions using the additional information contained in the polarization results.