The 3d-element transition metal dioxide MO(2), peroxide M(O(2)), and superoxide MOO clusters (M=Sc-Zn), are studied by density functional theory with the B1LYP functional. The reliability of the methods and basis sets employed was tested by a reinvestigation of the monoxides, for which a database of experimental data is available. The global minima on the M+O(2) potential energy surfaces corres...