Ice crystals larger than about 5 &mgr;m diameter were separated from interstitial particles in aircraft contrails and evaporated. Residual particles larger than 0.1 &mgr;m were analyzed by electron microscopy. Soot, metals, and volatile organic substances, apparently from the aircraft exhaust, were found. However, the residual particles also contained high percentages of minerals, thought to be crustal in origin, that were often mixed with sulfur. The percentage of particles in our samples (representing the larger residual particles from relatively large ice crystals) identified as exhaust-derived and the percentage apparently derived from the ambient aerosol were roughly equal, suggesting that ambient particles may be important in contrail formation. Possible explanations for this are presented. ¿ 1998 American Geophysical Union