VEGA-1 measurements of the plasma density and magnetic field in the coma of Comet Halley show characteristic signatures over a significant portion of the outbound pass. It is found that the assumption that there is a balance between the thermal and magnetic pressures in these features can be used to obtain estimates of the plasma temperature as a function of distance from the nucleus. These estimates indicate that the ions cool from -1.5~106 ¿K at 105 km to 2¿105 ¿K at 5¿104 km. The technique used here represents a novel approach whereby temperature measurements can be made in situations where only plasma density and magnetic field data are available. ¿ American Geophysical Union 1989