In the past decade, a variety of questions have been raised concerning the effectiveness of high-occupancy vehicle (HOV) lanes. In Southern California, recent evaluation studies on HOV lanes confirm the effectiveness of HOV lanes in several ways. However, little research has been performed to evaluate the air quality benefits of HOV lanes. This paper describes a study that examines operational differences in traffic dynamics between HOV lanes and mixed-flow (MF) lanes and evaluates their impacts on vehicle emissions. Four general HOV lane scenarios were identified: underutilized, neutral, well utilized, and overutilized. Extensive driving trajectories in both lane types for each scenario were collected. Their speed profile and joint speed-acceleration frequency distribution were analyzed and compared. Vehicle emissions and fuel consumption were then estimated with a state-of-the-art modal emissions model. The results show that HOV lanes produce lower emission rates per vehicle per mile in most cases, except when they are underutilized. When normalized by average vehicle occupancy, HOV lanes produce much lower emission rates per the same amount of travel demand, on the order of 10% to 70%. In almost every case, HOV lanes produce less emissions mass on a per-lane basis than MF lanes. Southern California freeway lane performance matrices show that on a typical weekday during the summer of 2005, HOV lanes operated mostly under Scenarios 1 and 2 during peak periods. Overall, they were well utilized about 14% to 17% of the time. According to the emissions estimates, the HOV lanes are considered effective in reducing vehicle emissions.