The light absorbing and light scattering properties of on-road vehicle exhaust particulate were determined as a function of traffic composition at the Allegheny Tunnel on the Pennsylvania Turnpike during August–September 1979. This study was one part of a comprehensive experiment aimed at the chemical and physical characterization of vehicle exhaust particulate. Paniculate light absorption was determined by the integrating plate method, while light scattering was measured with an integrating nephelometer. Mass-specific optical coefficients (at 500.0 nm) have been derived from regressions of the optical and mass emissions data against traffic composition. For diesel vehicles (predominantly heavy-duty, but also including diesel passenger vehicles) the absorption coefficient, b′ abs / M′, was found to be 5.13 ± 0.28 m 2 g −1, while the light scattering coefficient, b′ scat / M′, was 1.99 ± 0.07 m 2 g −1. Diesel vehicle emissions were responsible for greater than 90% of the light extinction in the tunnel, although diesels accounted for only 23% of the vehicle miles travelled. Estimates for b′ abs / M′ and b′ scat / M′ for particulate from gasoline-powered vehicles were 8 ± 16 m 2 g −1 and 6 ± 10 m 2 g −1, respectively, while the analogous values for ambient particulate were b abs / M< 1 m 2 g −1 and b scat / M = 5.0 ± 1.0 m 2 g −1.