Geometric structures of gas-phase palladium oxide cluster cations, PdnOm +, were investigated for stable compositions by ion mobility mass spectrometry (IMMS) and quantum chemical calculations. Pure metallic (m = 0) and oxygen-deficient (m < n) cluster cations were preferentially obtained from the mass spectra as a result of collision-induced dissociation. Structures of cluster series, Pd3Om + (m = 1-6), Pd4Om + (m = 2-8), and Pd5Om + (m = 3-8), were determined by comparing experimental collision cross sections obtained by IMMS and theoretical collision cross sections of optimized structures by density functional theory calculations. As for the Pd3Om + cluster cations, structural transition was observed from one-dimensional chains to two-dimensional (2D) branched/2D sheets and finally to three-dimensional (3D) compact structures with increasing m. These 2D and 3D isomers were found to retain their triangular metal-core configuration. 2D sheets and 3D compact isomers that maintain a tetrahedral metal-core configuration were assigned for the Pd4Om + cluster ion. Two structural isomers were assigned for Pd5Om +, one with a 3D square pyramidal metal-core configuration and another one with a 3D distorted pentagonal. Furthermore, the structures of oxygen-deficient cluster ions include atomic oxygen preferentially, whereas structures with molecular oxygen were commonly assigned for oxygen-rich (m > n) cluster ions.