Recently observed valence-shell ionization spectra of XONO 2 (X=F, Cl, Br, I) are investigated by the SAC-CI (symmetry-adapted-cluster configuration-interaction) method. This is the first attempt to give the quantitative assignments for these molecules by the reliable ab initio calculations including electron correlations. Calculated spectra agree very well with the experimental ones, especially for FONO 2 and ClONO 2 . New assignments are proposed for the primary peaks in the outer-valence region, which were characterized by the Koopmans theorem in the previous experimental work. It is also found that there are many shake-up states with considerable intensity in the inner-valence region, especially for BrONO 2 and IONO 2 . The theoretical ionization spectrum of IONO 2 is presented prior to the experimental observations.