Two-phase coil loop thermosiphon heat exchangers have been investigated both experimentally and by using a computer simulation program which was found to simulate the experimental results very closely. The effect of loop charge, tube diameter and length, angle of inclination of the coils and temperature difference on the loop conductance were investigated.It was found that the performance was very sensitive to loop charge, tube diameter and length but reasonably insensitive to the applied temperature difference and inclination angles greater than 15 deg. from the vertical.