Special Sensor Microwave Imager (SSM/I) observations in cloud systems are studied over the tropics. Over optically thick cloud systems, the presence of polarized signatures at 37 and 85 GHz is evidenced and analyzed with the help of cloud top temperature and optical thickness extracted from visible and IR satellite observations. Scattering signatures at 85 GHz (TbV(85)≤250 K) are associated with polarization differences≥6 K ~50% of the time over ocean and ~40% over land. In addition, over thick clouds the polarization difference at 37 GHz rarely is negligible. The polarization differences at 37 and 85 GHz do not stem from the surface but are generated in regions of relatively homogeneous clouds having high liquid water content. To interpret the observations, a radiative transfer model that includes the scattering by nonspherical particles is developed, based on the T matrix approach and using the doubling and adding method. In addition to handling randomly and perfectly oriented particles this model can also simulate the effect of partial orientation of the hydrometeors. Microwave brightness temperatures are simulated at SSM/I frequencies and are compared with the observations. Polarization differences of ~2 K can be simulated at 37 GHz over a rain layer, even using spherical drops. The polarization difference is larger for oriented nonspherical particles. The 85 GHz simulations are very sensitive to the ice phase of the cloud. Simulations with spherical particles or with randomly oriented nonspherical ice particles cannot replicate the observed polarization differences. However, with partially oriented nonspherical particles the observed polarized signatures at 85 GHz are explained, and the sensitivity of the scattering characteristics to the particle size, asphericity, and orientation is analyzed. Implications on rain and ice retrievals are discussed. ¿ 2001 American Geophysical Union