This study addresses whether high concentrations of small ice crystals in the upper 1 km or so of high, thick tropical cirrus clouds are principally responsible for the highly reflective clouds observed over the equatorial Pacific warm pool. This region of the tropics has recently been shown to contain extensive shields of cirrus clouds which significantly influence the global climate through their effect on the radiation budget of the tropics. In-situ and remote sensing measurements of cloud microphysical and radiative properties from field programs in the central and western tropical Pacific and radiative transfer calculations are used to derive distributions of cloud microphysical properties with height and their relationship to cloud radiative properties. Clouds associated directly with convection are shown to have sufficiently high optical depths near cloud top to produce localized areas of bright or optically thick cirrus, reflecting more than 40% of the incoming solar radiation. However, in general the upper parts of cirrus cannot alone account for the high albedos (fraction of incoming solar energy reflected) but do contribute substantially when high albedos are observed. The lower parts of the cirrus, in some cases extending down to the melting layer or below when they are called stratiform cloud regions, are usually necessary to produce high albedos. ¿ 1998 American Geophysical Union