European Stage III emissions requirements will be difficult to meet for diesel passenger cars if lean NO x catalysts are not available. Current prototype lean NO x 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°C. The latter has optimum activity at higher temperatures, usually above 350°C. Maximum flow reactor activity of 40–55% NO x 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 NO x has been emitted at inlet catalyst temperatures from 120 to 200°C. These temperatures fall below optimum temperatures for current Pt based lean NO x catalysts. For temperatures above 350°C, where Cu zeolite catalysts are active, one vehicle has emitted ca. 30% of mass NO x 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 (SO 2) and space velocity (SV) have been investigated as part of a strategy to optimize NO x removal with lean NO x catalysts. Elimination of feedgas SO 2 can lower NO x 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 SO 2 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 NO x removal over Pt by (i) lowering NO x light off temperature, (ii) lowering the temperature at which peak NO x conversion occurs, (iii) increasing the level of peak NO x conversion, and (iv) widening the temperature window for NO x reduction. For Cu zeolite, decreasing space velocity can help mainly by lowering NO x light off temperature and temperature where maximum NO x conversion starts. Both increased catalyst volume and sulfur removal provide Pt catalysts with a NO x temperature window that coincides better with low temperatures where most NO x mass is emitted. SV effects on lean NO x reduction are explained by discussion of possible mechanistic features.