gamma(L)-Bi2MoO6 (L low temperature phase) catalysts, whose surface compositions have a Mo/Bi ratio above = 0.5, exhibited high selectivity in the partial oxidation of C3H6, while catalysts with Mo/Bi surface ratios near or below = 0.5 exhibited low selectivity. gamma(L)-phase catalysts which have Mo/Bi surface ratios greater than = 0.5,were demonstrated to form beta-Bi2Mo2O9 on their surface. An interaction between the beta- and gamma(L)-phases was observed in these catalysts' UV-vis spectra at 430 nm. The new beta-phase material seems to grow along b-axis of gamma(L)-phase, i.e., perpendicular to MoO2-Bi2O2 layers. Structure visualizations revealed that the alpha-Bi2Mo3O12, beta- and gamma(H)-phases, which are selective catalysts, contain twin Mo tetrahedral structures, and that their Mo and Bi ions lie on the same plane. The pure gamma(L)-phase does not contain this structure. A model for the very rapid transfer of oxygen between the gamma(L)- and beta-phases is discussed in relation to the kinetics of C3H6 oxidation. (C) 2009 Elsevier B.V. All rights reserved.