Experimental natural convection Nusselt numbers are reported for a cubical, air-filled cavity that has one pair of opposing faces isothermal at different temperatures, T-h and T-c, the remaining faces having a linear variation from T-c to T-h. Average Nusselt numbers at the cold face are given for Rayleigh numbers Ra equal to 10(4), 10(5), 10(6), 10(7) and 10(8), and for three angles of inclination phi of the isothermal faces from horizontal: namely phi = 0, 45 and 90 degrees. The 95% confidence limits on the measured Nusselt number Nu are typically 1% of the Nu. Unexpectedly, two Nusselt numbers were found at phi = 0 degrees and Ra = 10(5), depending on the initial conditions. The results are intended to provide data for a recently-defined benchmark problem in CFD.