Emission rates of 22 gas-phase hydrocarbon compounds from motor vehicles in highway operation were determined in the Allegheny Mountain Tunnel of the Pennsylvania Turnpike with tenax adsorbent. Concurrently, particulate-phase components were collected on quartz-fiber filters. The emission rates of n-alkanes c8h18 through c17h36 and of phenol, toluene, ethylbenzene, propylbenzene, styrene, methylstyrene, ethynylbenzene, indan, indene, naphthalene, 1-methylnaphthalene, and 2-methylnaphthalene are given for heavy-duty diesel and light-duty gasoline-powered vehicles. Emission rates determined from this study are generally consistent with tailpipe emission rates determined from published dynamometer studies. The distribution of the n-alkanes between the gas and particulate phases appears to be related to vapor pressure; i.e., the gas/particulate mass ratio is approximately proportional to the vapor pressure in the limited range that could be tested (C14 through C17). Qualitative results from the same study, identifying (completely or partially) more than 300 gas-phase hydrocarbon species, were reported in a previous article. (Author/TRRL)