The paper gives results of a fundamental evaluation of an electronic air cleaner (EAC). The ozone generation rate and particle-size-dependent (0.01-10 micrometers) filtration efficiency of an in-duct residential EAC were measured. Filtration efficiencies were typically 70-90%, showing decreasing efficiency with increasing flowrate. Ozone generation rates were about 3 micrograms/s. Scans of the aerosol concentration on the downwind face of the EAC were used to locate, then eliminate, areas of aerosol sneakage. Sneakage was detected along the top and bottom of the EAC face, apparently due to incomplete aerosol charging for aerosol passing near the ends of the ionizing wires. Areas away from the top and bottom had near-zero aerosol penetration. Based on these results, the inlet to the EAC was masked to eliminate airflow through the sneakage areas. The resultant efficiency of the masked EAC was nearly 100% for particles larger than 0.1 micrometer diameter; however, the filtration efficiency for particles smaller than 0.1 micrometer was not significantly affected by masking.