A set of experiments was devised to investigate the effect of Prandtl number on the thermal performance of offset strip fins and to verify the extension of Kays and London air test data to liquids. All offset strip fin with a cross section of 6.25 cm X 0.938 cm (2.5 in X 0.375 in) and a flow length of 25.4 cm (10 in) was tested for water and ethylene glycol at Reynolds numbers from 30 to 3,000, and Prandtl numbers from 4 to 80. Results correlate according to Nu = 0.287 Re-0.58 Pr-0.5. For the conditions tested in the present work, the fin efficiencies were in the range of 5-25%. It is suggested that for liquid coolants or for asymmetric boundary conditions, a different fin geometry needs to be designed for higher efficiencies.