A series of experiments were conducted to investigate the effects of different working fluids on the behavior and performance of a bidirectional thermodiode, The thermodiode wits made Lip of two rectangular loops mounted between it collector plate and a radiator plate. The loops were filled with a working fluid for effective heat transfer when the thermodiode was forward biased. Five different working fluids were testec with thermal conductivity values ranging from 0.1 to 0.607 W/m-K, thermal expansion coefficient values ranging from 2.54 x 10 (4) to 1.43 x 10 (3) l/K, and kinematic viscosity values ranging from 6.5 x 10 (7) to 1 x 10 (4) m(2)/s. The thermodiode was heated by it radiant heater consisting of 88 halogen lamps that generated a heat flux of about 10(3) W/m(2) on the collector Surface. Experimental results indicated that the onset time for natural convection to be induced throughout the diode system did not differ considerably when different working fluids were used. On the other hand the required fluid temperature differences in the loops for the onset of throughflow were quite different and depended strongly Upon the viscosity and other properties Such as thermal expansion coefficient and specific heat of the working fluid. Of the five fluids tested, water and low-viscosity silicon oil bad the highest heat transfer rate. An analytical model wits developed to predict and analyze the steady operation of the diode system when different working fluids are used. Published by Elsevier Ltd.