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In-use vehicles which are high emitters make a large contribution to the emissions inventory. It is not known, however, whether high-emitting vehicles share common emissions characteristics. We study this by first examining laboratory measurements of second-by-second engine-out and tailpipe emissions from a small number of MY90-97 high-emitting vehicles. We distinguish high-emitter types by the behavior of six ratios in low- and moderate-power driving: the engine-out emissions indices (engine-out pollutant to fuel-rate ratios) and the catalyst pass fractions (tailpipe to engine-out ratios) for CO, HC, and NOx. Four general types of high emitter are observed: 1) fuel-air ratio excessively lean, 2) fuel-air ratio excessively rich, 3) partial combustion such as misfire, and 4) severe deterioration in catalyst performance in vehicles where malfunctions of Types 1, 2 or 3 are not predominant. We also find that these behaviors may be chronic, or may only occur transiently.