INTRODUCTION Scott Research Laboratories^ Inc.@ tested and evaluated 17 instruments which could possibly be used for the measurement of automobile exhaust emissions. This report presents the test results and discussion of that evaluation. The work was done under TRW subcontract AK 727 SC in support of CRC/APRAC Project CAPE 13-68@ Emissions Test Program. The overall program (CAPE 13-68) is designed to define the degree to which properly implemented inspection and maintenance procedures would effectively reduce and control vehicle exhaust emissions. In addition@ the study should contribute to the development of optimum inspection and maintenance procedures applicable for those vehicle parts and components which affect exhaust emissions. The instrument evaluation portion of this program was an effort to determine the suitability of presently available commercial instruments for the measurement of automotive vehicle exhaust emissions. In addition to regulatory emission criteria imposed on the manufacture of new vehicles@ some states have already taken steps to institute exhaust emission inspection procedures in conjunction with their periodic (e.g.@ New Jersey) or random (e.g.@ California) motor vehicles safety inspections. Because of the ever-increasing awareness of air pollution@ increased attention is being directed to more universal and sophisticated vehicular emission inspections. This will require newly developed measurement procedures which can be both quickly and easily performed. The instruments used for measurement must provide reliable diagnostic evidence of exhaust characteristics and be otherwise suitable for their intended use. These factors provided a basis for the evaluation of the performance of the instruments included in this study.