In this study we present the microphysical characteristics of 21 jet contrail clouds sampled in situ and examine the possible effects of exhaust on natural cirrus and radiative effects of contrails. Microphysical samples were obtained with Particle Measuring Systems (PMS) 2D-C, 1D-C, and FSSP probes. About one half of the study contrails were generated by the sampling aircraft, a Cessna Citation, primarily at times of 3--15 min after generation; the source and age of the others is unknown. On average, the contrails contained particles of mean diameter of the order of 10 μm in concentrations exceeding 10,000 L-1. Contrails embedded in natural cirrus appeared to have little effect on the natural cloud microphysics. Anomalous diffraction theory was used to model radiative properties of sampled contrails. The contrail cirrus showed considerably more spectral variation in extinction and absorption efficiencies than natural cirrus because of the large numbers of small crystals in contrails. Embedded contrails also displayed greater emissivity and emission than natural cirrus and a greater spectral variation in transmission. ¿ 1999 American Geophysical Union