Natural convection experiments were performed with aluminum oxide microparticle aqueous suspensions in thin enclosures of circular planform at angles of inclination to the horizontal of 90°, 30° and 0°. The average size of the aluminum oxide particles was about 250 nm, and volume fractions of 1.31%and 2.72%were used. The aspect ratio varied from 50.7 to 10.9, and the maximum Raleigh number was 3 × 10⁵. No effect of particles on the Nusselt number–Rayleigh number relation was found for the vertical enclosure at 90°. However at 30° and 0° (horizontal) there was a decrease in Nusselt number compared to pure water, which was pronounced at lower Rayleigh number and higher particle concentrations. This anomalous behavior is attributed to sedimentation.