Very tiny Au particles on TiO2 show excellent activity and selectivity in a number of oxidation reactions. We have studied the vapor deposition of Au onto a TiO2(110) surface using XPS, LEIS, LEED and TPD and found that we can prepare Au islands with controlled thicknesses from one to several monolayers. In order to understand at the atomic level the unusual catalytic activity in oxidation reactions of this system, we have studied oxygen adsorption on Au/TiO2(110) as a function of Au island thickness, and have measured the titration of this adsorbed oxygen with CO gas to yield CO2, as function of Au island thickness, CO pressure and temperature. A hot filament was used to dose gaseous oxygen atoms. TPD results show higher O-2 desorption temperatures (741 K) from ultrathin gold particles on TiO2(110) than from thicker particles (545 K). This implies that O-a bonds much more strongly to ultrathin islands of Au. Thus from Bronsted relations, ultrathin gold particles should be able to dissociatively adsorb O-2 more readily than thick gold particles. Our studies of the titration reaction of oxygen adatoms with CO (to produce CO2) show that this reaction is extremely rapid at room temperature, but its rate is slightly slower for the thinnest Au islands. Thus the association reaction (COg + O-a --> CO2,g) gets faster as the oxygen adsorption strength decreases, again as expected from Bronsted relations. For islands of about two atomic layers thickness, the rate increases slowly with temperature, with an apparent activation energy of 11.4 +/- 2.8 kJ/mol, and shows a first-order rate in CO ressure and oxygen coverage, similar to bulk Au(110).