Aberration-corrected scanning transmission electron microscopy at the sub-ngstrom resolution allows imaging the structure of catalytic materials at the single atom level and permits fundamental studies of the behavior of heavy metal catalytic species as a result of elevated temperature gas-treatments. The present study is aimed at understanding the development of clusters and nanoparticles of Pt on gamma-alumina during reduction treatments of a pre-oxidized highly dispersed catalyst. A special built ex situ reactor and a specimen holder allowing cyclic anaerobic transfer between the reactor and microscope were used for the study. The number of atoms in a nascent cluster can be determined along with the general shape of the cluster. Reduction experiments without air exposure of the sample showed that although clusters are formed at 500 A degrees C, many Pt atoms are not associated with the cluster and are still dispersed on the catalyst support. After a 700 A degrees C reduction, all of the Pt atoms are associated with the clusters. Movement of the clusters on the catalyst support is different depending upon the catalyst support. Aberration-corrected scanning transmission electron microscopy at the sub-ngstrom resolution allows imaging the structure of catalytic materials at the single atom level and permits fundamental studies of the behavior of heavy metal catalytic species as a result of elevated temperature gas-treatments. The present study is aimed at understanding the development of clusters and nanoparticles of Pt on gamma-alumina during reduction treatments of a pre-oxidized highly dispersed catalyst. A special built ex situ reactor and a specimen holder allowing cyclic anaerobic transfer between the reactor and microscope were used for the study. The number of atoms in a nascent cluster can be determined along with the general shape of the cluster. Reduction experiments without air exposure of the sample showed that although clusters are formed at 500 A degrees C, many Pt atoms are not associated with the cluster and are still dispersed on the catalyst support. After a 700 A degrees C reduction, all of the Pt atoms are associated with the clusters. Movement of the clusters on the catalyst support is different depending upon the catalyst support.