Due to the excellent catalytic performance of manganese oxide (K-OMS-2) in a wide range of applications, incorporation of various dopants has been commonly applied for K-OMS-2 to acquire additional functionality or activities. However, the understanding of its substitution mechanism with respect to the catalytic performance of doped K-OMS-2 materials remains unclear. Here we present the structural distortion (from tetragonal to monoclinic cell) and morphological evolution in K-OMS-2 materials by doping hexavalent molybdenum. With a Mo-to-Mn ratio of 1:20 (R-1:20) in the preparation, the resultant monoclinic K-OMS-2 shows a small equidimensional particle size (similar to 15 nm), a high surface area of 213 m(2)g(1), and greatly improved catalytic activity toward CO oxidation with lower onset temperatures (40 degrees C) than that of pristine K-OMS-2 (above 130 degrees C). HR-TEM analyses reveal direct evidence of structural distortion on the cross-section of 2 x 2 tunnels with the absence of 4-fold rotation symmetry expected for a tetragonal cell, which are indexed using a monoclinic cell. Our results suggest that substitution of Mo6+ for Mn3+ (rather than Mn4+) coupled with the vacancy generation results in a distorted structure and unique morphology. The weakened MnO bonds and Mn vacancies associated with the structural distortion may be mainly responsible for the enhanced catalytic activity of monoclinic K-OMS-2 instead of dopant species.