Niobium compounds and materials such as niobium pentoxide (Nb2O5 . nH(2)O), niobium phosphate (KNbPO4), niobium layer compounds (K4Nb6O17, HCa2Nb3O10, etc.) and mixed oxides containing niobia (Nb2O5-SiO2, Nb2O5-Al2O5, Nb2O5-TiO2, Nb2O5-V2O5, etc.) are shown to be useful as promoters and supports of catalysts and as unique solid acid catalysts, selective oxidation catalysts and photosensitive catalysts. High catalytic activities, selectivities and stabilities of these niobium compounds and materials and metals (Rh, Pd, Ni, Co, etc.) supported on Nb2O5 are demonstrated for various reactions given in the following list of contents.