European Stage III emissions requirements will be difficult to meet fur diesel passenger cars if lean NOx catalysts are not available. Current prototype lean NOx technology for diesels consists of Pt based and Cu zeolite catalysts. Both types have been examined for this study. The former is most active at low temperatures, approximately 190-250 degrees C. The latter has optimum activity at higher temperatures, usually above 350 degrees C. Maximum flow reactor activity of 40-55% NOx conversion (25 000-50 000/h space velocity) has been measured for Pt catalysts using a synthetic feedgas. During the MVEuro2 driving cycle, 35-40% of mass NOx has been emitted at inlet catalyst temperatures from 120 to 200 degrees C. These temperatures fall below optimum temperatures for current Pt based lean NOx catalysts. For temperatures above 350 degrees C, where Cu zeolite catalysts are active, one vehicle has emitted ca. 30% of mass NOx during MVEuro2, These high temperatures are achieved during high speed, hard acceleration driving; although attained briefly during the MVEuro2 cycle, these high temperature emissions could be a critical contribution under customer driving conditions. Effects of sulfur dioxide (SO2) and space velocity (SV) have been investigated as part of a strategy to optimize NOx removal with lean NOx catalysts. Elimination of feedgas SO2 can lower NOx light off temperature for both Pt and Cu zeolite. Some Pt catalysts do not show this behavior. Additional evaluation of a Cu zeolite catalyst demonstrates that poisoning by feedgas SO2 is reversible during evaluation or aging. This result suggests that if sulfur could be removed from diesel fuel, aged Cu zeolite catalysts could be practical. Decreasing space velocity will help NOx removal over Pt by (i) lowering NOx light off temperature, (ii) lowering the temperature at which peak NOx conversion occurs, (iii) increasing the level of peak NOx conversion, and (iv) widening the temperature window for NOx reduction. For Cu zeolite, decreasing space velocity can help mainly by lowering NOx light off temperature and temperature where maximum NOx conversion SV effects on lean NO, reduction are explained by discussion of possible mechanistic features.2?