Eight catalytically equipped vehicles were used to compare the effects of an ashless, nonphosphorus lubricant and a conventional SE lubricant on the deterioration of two different types of bead catalyst operating in field service. Composite CO emissions (measured by the 1975 Federal Test Procedure) approximately doubled during the 25,000-mile test with both oils relative to the fresh catalyst. No significant increase in composite HC emissions was shown. There were no significant differences between the two oils with respect to either HC or CO emissions at the end of the test. Analysis of the aged catalyst showed average phosphorus contamination of 0.07 wt % with the conventional oil compared to 0.01 wt % with the ashless oil. The higher level of phosphorus contamination with the conventional oil was not accompanied by an increase in catalyst deterioration relative to the ashless oil.