The laminar natural convection in an air-filled square cavity with a partition on the top wall was experimentally investigated. Temperature measurements and flow visualizations were performed for cases with heated and cooled vertical walls (corresponding to a global Grashof number Gr(H) of approximately 1.3 x 10(8)) and non-dimensional top wall temperatures theta(T) of 0.56 (insulated) to 2.3. Experiments were performed with an aluminum partition with non-dimensional height H-P/H of 0 0625 and 0 125 attached to the top wall at x/H = 0.1, 0 2, 0 4 and 0.6. The blockage effect and/or the thermal effect of the partition resulted in changes to the temperature and flow fields. but were mainly limited to the vicinity of the partition. The partition oil the heated top wall resulted in a recirculating flow between the partition and the heated vertical wall. For a given partition height. the structure of this recirculating now was dependent or the partition location and theta(T). A thermal boundary layer developed along the rear surface of the partition due to the thermal effect of the partition The ambient temperature Outside the boundary layer and Nu near the corner region was affected by the partition height due to the change in the recirculating now and due to the thermal effect on the rear surface of the partition. (C) 2010 Elsevier Ltd. All rights reserved.