In an ideal, 3-D, closed system that experiences compression and expansion, the temperature (T) and density (n) of a monatomic gas are related adiabatically through the polytropic law; i.e., T∝n&agr;-1, here &agr; is the pmlytropic index and equals 5/3. This relationship has been examined over a variety of scale sizes in the solar wind, and empirical measurements of &agr; vary a great deal. As shown in this paper, blanket applications of the polytropic law to ambient solar wind can return misleading measurements of &agr;, due to inhomogeneities in the solar wind originating at coronal source regions. However, on a smaller and more localized scale, adiabatic compression and expansion is observed. In this paper, we measure &agr; by examining parcels of originally homogeneous material that have become compressed at stream interactions. A superposed epoch analysis of 73 isolated stream interactions observed by PVO illustrates this adiabatic behavior. Observations of individual stream interactions show clear intervals where &agr;~5/3 and also a few cases where &agr;~2, suggesting that the number of degrees of freedom may occasionally be restricted.¿ 1997 American Geophysical Union