Admission of carbon monoxide to reduced Pd/NaY at room temperature induces substantial migration and coalescence of primary Pd clusters, as evidenced by EXAFS. In samples calcined at T_C= 500 °C and reduced at T_R= 200 °C, isolated Pd atoms in sodalite cages prevail initially; Pd clusters in supercages with Pd coordination numbers of 2.4 or 4 are generated by the pretreatment programs T_C= 250 °C/T_R= 200 °C or T_C= 500 °C/T_R= 350 °C, respectively. In all these cases Pd₁₃ carbonyl clusters prevail after admission of CO at room temperature. The growth of Pd nuclei is rationalized by assuming that, before CO is admitted, bare Pd interacts with the zeolite matrix, e.g. via proton or Na⁺ bridges linking Pd with cage-wall oxygens. Adsorption of CO weakens this interaction as primary Pd carbonyl clusters are formed. The mobile Pd carbonyl clusters will coalesce in the supercages until further transport is sterically restricted. This process leads to prevailing Pd₁₃ carbonyl clusters at room temperature as the Pd core of 8.2 Å is unable to traverse the 7.5 Å window. At elevated temperatures, e.g. 200 °C, further growth is possible as the supercage windows expand. As a result, Pd₄₀ clusters are formed and completely fill their supercage.