Diesel exhaust is typically at lower temperature compared to gasoline exhaust and would need a catalyst that has activity in low temperature range to be effective. Hence considerable research has been directed to improve low temperature activity of catalysts used in diesel application. One of the aspects that has been widely reported in literature is that small Pt clusters have a positive effect on reducing the CO light off temperature (LOT). To examine this phenomenon closely, the present work was taken up to correlate Pt cluster size with performance. Catalysts were prepared on various supports - Alumina, Siliceous clay, ceria-zirconia, etc with different metal loadings and the calcinations conditions were varied both in time and temperature as well as calcinations atmosphere. The cluster sizes were ascertained using Phillips Tecnai 20 Transmission Electron Microscope. The performance was studied both under simulated conditions in a laboratory reactor as well as on a chassis dynamometer for mass emissions.The Results suggest that in Non-O2 Containing Atmosphere (such as N2, CO), the Pt cluster size is as low as 1-2.5 nm where as in O2 containing atmosphere, the cluster size is around >8 nm. The lower cluster size had a positive effect on CO LOT where a benefit of about 20 degrees could be achieved. However on aging, this advantage of reduced LOT disappeared. This suggests that the small clusters formed by CO & N2 calcination are unstable and agglomerate into bigger clusters. However, it was found that addition of Pd as reported in literature, this advantage due to CO and N2 calcination could be retained. The addition of Pd is just for stabilizing the Pt clusters.The reason for smaller clusters in CO & N2 calcined samples seems to be lower mobility of Pt particles during calcination. If the samples are dried in microwave oven and calcined in air, the cluster size is relatively smaller (5 nm) but still much larger than CO & N2 calcined samples. When both these results are interpreted together, it appeared that mobility of Pt should be minimized during both drying and calcination steps to get smaller clusters and hence lower LOT.Lowering the calcination temperature (350 °C, 450 °C) while helped in having relatively smaller clusters (6-7 nm), the number of clusters, particularly at 350 °C is low. Lowering the calcination time also had similar effect.The reactor and Mass emission data suggests that CO and N2 calcined samples are indeed better than air calcined samples. Also, the CO and N2 calcined samples behave similarly.