This report describes the design features and development testing of a dilution tunnel for sampling automotive sulfate emissions. The design of the dilution tunnel and sampling system was the result of a concensus of ideas from representatives of the EPA and industry. The development testing included a propane traverse to verify exhaust mixing; testing the critical flow sampling system; development and use of a sulfate mist generator to determine system sulfate recovery; and evaluation of system losses using automotive exhaust. The propane traverse demonstrated acceptable exhaust gas mixing. While system losses were acceptable using the acid mist generator, the system was found to have unacceptable sulfate losses at tunnel temperatures above 85C (185F) when tested with automotive exhaust. At the completion of the development tests, the sampling system, including the tunnel and sampling and control console was delivered to EPA, Ann Arbor. A complete set of drawings of the tunnel was furnished with the tunnel.