Considering the solar-wind plasma as fully ionized, electrically neutral and radially expanding spherically symmetric, we solve the equations of mass and energy conservation. The plasma under consideration is assumed as one-fluid and its parameters are estimated according to the measured data at 1 AU during the period of the observed very low temperatures. In this way, we could obtain an analytical relation between the temperature and the velocity of the solar-wind plasma. Solar-wind velocity and temperature observations, made aboard the Vela 5, 6 and IMP 6 satellites during the period August 1969–May 1974, suggest that on the average a solar-wind velocity decrease correlates with a proton and electron temperature depression. This depression could possibly originate from the expansion of the solar wind between a fast-moving plasma stream and a following slower one. The above correlation of data, which is also in accordance with the conservation of energy in the solar wind, is compared with the analytically derived relation. It seems, at least within the limits of errors, that the theoretical relation fits the experimental observations. Consequently, we could propose that the observed very low temperatures in the solar wind may be due to the fact that the temperature decrease of the expanding plasma exceeds the temperature increase due to the heat conduction of the electrons from the hot corona to the plasma along the open interplanetary-magnetic-field lines, so that on the whole the plasma is cold.