The capability of producing additional thrust along with the ability of vectoring the thrust makes the ejector an attractive device for V/STOL aircraft applications. In addition, the acoustic shielding provided by the walls of the shroud, and lowered exhaust velocities and temperatures at the ejector exit offer important advantages in the area of nozzle exhaust cooling. One of the physical processes which is known to play a large role on the performance of the total system is the mixing between the hot primary and cold secondary streams. Thus an attempt is made here to investigate the mixing process of a hot rectangular jet with an entrained cold ambient fluid in a simple form of a ejector. Primary jet heating, over the temperature range tested, has no significant effect on the velocity field within the ejector shroud. The temperature decay within the ejector shroud displayed differences in comparison with the free jet up to a location where the primary jet spreads to the mixing duct wall. Downstream of this location the centerline temperature within the ejector is higher than that of a free jet. A theoretical analysis is made to set up a determinate mathematical problem for the flow through a constant area ejector. It is shown that multiple solution (more than two) occur, and admissible ones out of these are those that fulfill the entropy condition.