Multichannel radiometer measurements of a newly formed contrail taken from a NASA ER-2 aircraft over northern Oklahoma during the FIRE Cirrus-II experiment are used in a split-window retrieval of the contrail's particle size and optical depth. The effect of horizontal inhomogeneity in an underlying cirrus layer on the contrail retrieval is studied through the use of multi-dimensional radiative transfer model simulations. Estimates of the contrail's effective radius are significantly smaller (5--7 &mgr;m) than those in the underlying cirrus (21 &mgr;m). The effect of horizontal inhomogeneity in the underlying cirrus on contrail particle size and optical depth retrievals is generally smaller than the corresponding effects of particle shape or spectral variations in surface emissivity on the retrievals. The differences in particle size retrievals are 10 percent or less as a result of the inhomogeneity, while they are from 10 to 40 percent for ice cylinders, and range from 1 to 40 percent for ice spheroids. Horizontal inhomogeneity affected the optical depth retrievals by only one or two percent, while surface emissivity variations affected the retrievals by 12 to 24 percent, and particle shape produced differences as large as 9 percent. ¿ American Geophysical Union 1996