Transition-metal dioxide anions RhO2-, IrO2-, PtO2-, and AuO2- were produced from cocondensation of laser-ablated metal atoms and electrons with dioxygen in excess argon at 6 K. Photosensitive absorptions were assigned to the antisymmetric stretching vibration (v(3)) of the metal dioxide anions from isotopic shiftsand splittings as well as theoretical frequency calculations. On the basis of the observed v(3) vibrational frequencies for (MO)-O-16, and M18O2, the anions are estimated to be linear or near linear. Density functional calculations at the DFT/B3LYP level predicted that the third-row transition-metal dioxide anions IrO2-, PtO2-, and AuO2- have sinalet, doublet, and triplet ground states with linear structures. while the RhO2- anion was predicted to have a slightly bent geometry.