It is conceivable that the high D2 desorption onset temperature during C2D4 TPD partly results from strengthening of Pt-D binding due to the presence of carbonaceous co-adsorbates on the clusters, and this possibility was tested by subjecting a Pt4/alumina sample to two C2D4 TPD runs prior to the D2 TPD experiment, depositing ~4 C atoms/Pt4. As shown by the "Pt4 post 2 C2D4 TPD" data, both the D2 signal intensity and desorption temperatures decreased substantially, implying that co-adsorbed carbon actually weakened Pt-D binding, and also reduced the number of sites capable of supporting 150 K D2 adsorption.