Relationships between light extinction coefficients, visual range, and aerosol mass and composition were studied in Houston, TX. Light extinction coefficients, measured with a telephotometer and black-box targets, agreed accurately with sums of light scattering and absorption coefficients. The light-scattering coefficient due to particles in heated air was highly correlated (R=0.987) with the mass concentration of fine particles (É462.5 mum), and the ratio of averages was 3.5 m^2g-1. Major species in the fine fractiulfate and related cations; the average sulfate was predominantly ammonium sulfate mixed with 19per thousand sulfuric acid during the day and 7per thousand sulfuric acid at night. The coarse fraction (2.5-15 mum) was alkaline and consisted of 69per thousand crustal matter, 12per thousand carbon, and 7per thousand nitrate. The average daytime light extinction coefficient was apportioned 32per thousand to sulfate and related cations, 17-24per thousand to carbon, 16per thousand to water on particles, 6per thousand to Rayleigh scattering, 5per thousand to nitrogen dioxide, and 13 ± 7per thousand to unmeasured matter.