Steady-state rates of heat dissipation under natural-convective conditions, from heat exchangers with vertical rectangular fins attached to a rectangular base, have been measured. The fins were relatively small, with a length of 150 mm and a protrusion of either 10 or 17 mm. The tests were carried out with the base of the fin array at either 20 or 40 K above the mean temperature of the environmental air in the laboratory. The optimal inter-fin separations (for achieving maximum heat-transfer rates) were 9·0 ± 0·5 mm and 9·5 ± 0·5 mm for the vertical fins protruding outwards from the vertical base and upwards from the horizontal base respectively. The experimental results are presented as non-dimensional correlations.