Observations of marine stratocumulus obtained by three research aircraft during the JASIN experiment are presented. Detailed measurements were made of the thermodynamic, cloud physics and radiation fields. These showed an essentially well-mixed boundary layer with cloud liquid water contents close to their adiabatic values. The mean drop radius increased steadily towards cloud top, where evidence of inhomogeneous mixing was found. Both the longwave and shortwave radiative components of the cloud layer energy budget were measured. Good agreement was obtained between the observations and several radiation schemes and in particular the measured cloud shortwave absorption was close to the theoretical values. At midday, the net longwave cooling of the cloud layer was found to be approximately compensated by the shortwave warming, although the regions of net warming and cooling were separated vertically, thereby promoting convection within the cloud. The wider implications of these results to studies of stratiform cloud are discussed.