Aerosol absorption measurements using diffuse reflectance and transmission techniques were made as part of the First International Workshop on light absorption by aerosol particles during July and August 1980. Our diffuse reflectance measurements were used to determine the imaginary component of the refractive index and the mass absorption coefficient for the aerosols and for the bulk material from which the aerosols were generated. Mass absorption coefficients measured by these techniques appear to be lower than those determined by diffuse transmission techniques. Our diffuse transmission measurements of sigma(A), the volume absorption coefficient, show that the measured response for this technique will be dependent on filter orientation and on the type of filter. Our data suggest that there may also be a dependence on the physical properties of the aerosol. Average omega values calculated on the basis of these diffuse transmission measurements range from 0.18 for soots to 0.98 for ammonium sulfate.