The direct interaction of YBa2CU3Ox (x=6-7) with Co, H-2+CO, and NO is studied using x-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and simultaneous thermal analysis. The CO interaction results in the release of CO2 at 400-800 K, and the formation of BaCO3, as well as the reduction of Cu(II) and Y(III), at temperatures above 800 K. The interaction between YBa2CU3Ox and H-2+CO results mainly in the formation of gaseous CO2 and H2O at temperatures below 830 K. At temperatures around 830 K, Cu(II) is reduced to Cu(I). In addition to barium carbonate, the methoxy group as well as the hydroxyl group can be identified on the YBa2CU3Ox sample. At 1170 K carbides can be found on the sample surface. The NO interaction occurs at room temperature and above, leading to the release of N2, NO2, and N2O, as well as the formation of the nitride, nitrate, nitrite (NO2-), and nitrito (-ONO) groups. The nitride and nitrate species are mainly observed between 300 and 540 K, while at 830-920 K, barium nitrite is the predominant nitrogen-containing species on the surface. An active center (OBasquareCuO) and mechanisms for the interaction between YBa2Cu3Ox and the gases are postulated and applied to explain the high catalytic activity of YBa2CU3Ox.