An exhaust gas purifying catalyst must be durable, i.e., it must maintain a sufficient catalytic performance even after thermal degradation. Therefore, large amounts of platinum group metals (PGMs), such as Pt, Pd, and Rh, should be loaded onto the catalyst substrate. Exhaust gas heat deteriorates the catalyst by sintering the PGM particles, which decreases the active surface area. It is important to reduce the PGM load and many researchers have therefore attempted to carry out PGM load reduction while maintaining sufficient durability.We found that Pt ions could form Pt-hydroxide clusters in a hexahydroxyplatinate (IV) (Pt(OH)6·H2O) nitric acid solution. The Pt-hydroxide cluster size could be controlled by varying the Pt and nitric acid concentrations and solution temperature. We expected that these “larger Pt-hydroxide clusters” would be efficient at improving the durability because a larger cluster size in the solution would equate to a greater distance between each Pt particle or uniform distribution of Pt particles on the support material. The greater distance or uniform distribution would lower the opportunity for Pt particle association at high temperatures. Therefore, we investigated the catalytic performance of a Pt catalyst containing larger Pt-hydroxide clusters.The Pt catalyst containing larger Pt-hydroxide clusters exhibited an improved catalytic activity after thermal duration. Transmission electron microscopy (TEM) images revealed that the Pt particle distribution was more uniform on the Pt catalyst containing the larger Pt-hydroxide clusters than on the conventional catalyst. This uniformity is assumed to be the reason for the improved durability.