Airborne data are presented on the impact of cloud processing on the aerosol mass light-scattering efficiency. The measurements. on marine stratocumulus, suggest that cloud processing significantly enhanced the mass light-scattering efficiency in three of the five cases analysed. Enhancements were of the order of 10% for air detraining from the cloud deck relative to non-detraming air. A diagnostic modelling analysis suggested that the observed enhancements were consistent with the previously proposed explanation of in-cloud sulfate production in the particle size range for efficient light scattering.