The structures of Mo supported on alpha-, gamma-, and chi-aluminas were analyzed by ESCA, XRD, IR, and XAFS techniques, and the relationship between the structure of Mo and its catalytic activity for the epoxidation of allyl alcohol was discussed. The ranges of the Mo-monolayer formation as estimated by the ESCA technique were less than 18, 15, and 1 wt% of Mo-loading for gamma-, chi-, and alpha-aluminas, respectively. However, MoO3 was detected by an XRD analysis at 10 wt% of Mo-loading for gamma- and chi-aluminas; the ESCA technique overestimated the range of the monolayer. The monolayer on gamma- and chi-aluminas as apparently determined by the ESCA analysis consisted of isolated tetrahedral Mo, octahedral polymolybdate, and a small amount of MoO3. The monolayer on a-alumina contained only tetrahedral Mo-species. MoO3 was predominantly formed after the completion of the monolayer on all aluminas. The epoxidation activity of Mo was low in the monolayer region; especially, isolated tetrahedral Mo was completely inactive. MoO3 was the most active species for the epoxidation, and peroxo-type oxygen bound to Mo was deduced to be the active species.