Measurements of oxygen storage capacity (OSC) and HC conversion efficiency for 17 catalysts were carried out in the laboratory. All catalysts with steady state HC efficiency below 90% were found to have roughly equivalent and very low capacities to store oxygen. However, catalyst oxygen storage capacity was seen to rise sharply with HC conversion efficiency in excess of 90 percent. These results parallel the trends which are observed between rear HEGO/EGO indexes for OBD-II catalyst monitoring and HC conversion efficiency. In addition, temperature programed reduction (TPR) was found to lend insight into the relationship between catalyst OSC and HC conversion efficiency by providing a qualitative understanding of the mechanisms by which OSC deteriorates. TPR profiles showed that most of the usable oxygen storage is derived from surface ceria which is interacted with precious metals. In-use vehicle aging to 10K miles degraded steady state HC efficiency very little but was capable of partially breaking the precious metal-ceria interaction and greatly reduced oxygen storage capacity. More long term in-use vehicle aging (100K miles) was found to completely break the precious metal-ceria interaction and resulted in catalysts with very low capacities to store oxygen.