Measurements of the specific entropy argument of the solar wind protons, T/n&ggr;-1, reveal that nearly every occurrence of a high-speed stream seen at Ulysses in 1992-1993 is characterized by an abrupt interface at its trailing edge. These observations, made by the solar wind plasma instrument (SWOOPS), at a heliocentric range of 4.5 to 5 AU show that there is a discontinuous drop in specific entropy at the interface from a high value in the high-speed wind to a lower value in the slow interstream wind. This interface is coincident with, but much more abrupt than, compositional changes measured by the Solar Wind Ion Composition Spectrometer (SWICS) [Geiss et al., 1995>. These results suggest that a relatively thin interface can be identified which separates two plasmas of distinctly different origins as determined by the compositional measurements. A superposed epoch analysis performed on seventeen events reveal the interface is characterized by (1) an abrupt drop in entropy by a factor of ~1/3 due to an enhancement in density along with gradually declining temperature, (2) a distinct drop in the alpha/proton ratio from a value of ~5%, typical of the fast wind, to ~4% characteristic of the slow solar wind, and (3) relative changes in Mg10+/O6+ at the interface which are as large as the variations in the total Mg/O ratio and the freezing-in temperature derived from O7+/O6+. The specific entropy argument, a combination of commonly measured solar wind parameters, gives a strong signature of the trailing edge interface which is preserved as far out in the heliosphere as 5 AU and may provide useful information regarding the coronal origin of solar wind streams. ¿ 1999 American Geophysical Union