SOlar wind proton temperatures are measured simultaneously by the Voyager 1 and 2 spacecraft, far from Earth, and the IMP 8 spacecraft in Earth orbit. This technique allows the separation of radial and temporal variations of solar wind parameters. The average value of the proton temperature from 1 to 9 AU is observed to decrease as r-0.7¿0.2 which is slower than would be expected for adiabatic expansion. A detailed look at the solar wind stream structure shows that considerable heating occurs at the interface between high and low speed streams.