X‐ray photoemission results make it possible to compare Ti adatom reactivity with clean surfaces of CuO, Y2BaCuO5, and YBa2Cu3O7−x. Changes in the Ti 2p1/2,3/2and O 1score level emission during overlayer growth are related to surface reactions between the Ti adatoms and oxygen atoms leached from the substrates. The results show the growth of a TiO2‐like reaction species at initial Ti depositions, the formation of Ti suboxides at intermediate coverages, and the onset of metallic behavior at higher Ti coverages. The Ti‐O reaction products grow in a layer‐by‐layer fashion initially, but Ti metal overlayer growth occurs as clusters on this Ti‐O reacted region. Substrate modifications associated with Ti‐O reactions result in significant changes in the chemical environments of Cu, Ba, and Y for Y2BaCuO5and YBa2Cu3O7−x.