Measurements of the 2&pgr;-sr (hemispheric) diffuse-sky, direct, and total (diffuse and direct) transmitted solar flux were made at Boulder, Colorado, during the passage of the Mount St. Helens dust cloud on May 19 and June 3, 1980. The measurements were both narrow-band and broadband integrated over the solar spectrum. Verification of the presence of the cloud and its vertical profile was obtained by lidar. Analysis of the flux measurements by a perturbation technique yielded volcanic cloud optical properties useful for studies of climate variation. The ratio of backscatter to total scatter, imaginary term of the complex refractive index, albedo of single scatter, and optical depth are determined from the measurements. Estimates of solar radiation absorption and backward and forward scattering by the cloud are given. Certain radiative characteristics of the Agung volcanic cloud and the Mount St. Helens volcanic cloud are compared.