Empirical evidence is presented that solar wind thermal electrons obey a polytrope law of the form P=&Sgr;n&ggr; with polytrope index &ggr;=1.175¿0.03(3&sgr;). The Voyager 2 and Mariner 10 data used span the radial range from 0.45 to 4.76 AU and have a large dynamic range in density (four decades), and in temperature (over one decade), which is crucial for an unambiguous determination of &ggr;. The polytrope index &ggr; was found to be insensitive to systematic dependence on the bulk velocity. We find that there is a large ~¿50% variation in the entropylike stream tube constant &Sgr; relative to its best fit value &Sgr;0. For the Voyager 2 data set, which has 5831 hourly averages, this variaion represents a nearly random statistical sample of log &Sgr; about log &Sgr;0, thereby reducing systematic errors. A positive correlation on average between &Sgr; and bulk speed is found, which does introduce systematic errors (~5%) in the determination of &ggr; resulting in larger uncertainties than implied by a simple regression of log Te versus log n. Our results support the theoretical predictions by Scudder and Olbert (1979b) that solar wind thermal electrons in the asymptotic solar wind should obey a polytrope law with polytrope index &ggr;=1.16¿0.06(3&sgr;). When restricted to high-speed flows, these data do not support the suggestion made by Feldman et al. (1978) that &ggr;=1.45¿0.15(3&sgr;). We attribute this difference to the biased sample of stream tube constants &Sgr; encountered in that limited study and/or the smaller dynamic range of density and temperature available to the 1-AU observer. Our results support (1) the widespread impressions in the literature that solar wind electrons behave more like an isothermal than an adiabatic gas; (2) the arguments put forward by Scudder and Olbert (1979a,b) that Coulomb collisions are the dominant stochastic process shaping observed electron distribution functions in the solar wind, and (3) the assignment of the interplanetary potential as equal to approximately 7 times the random energy of the thermal electrons.