The results of an experimental study of laminar natural convection heat transfer in a square enclosure using air as the medium and having differentially heated isothermal Vertical walls and adiabatic horizontal walls are reported. The study has been carried out using a differential interferometer (DI). After many trials, the authors have been able to achieve nearly adiabatic boundary conditions for top and bottom walls of the enclosure, in the laboratory environment. The study provides Valuable information for many researchers - experimentalists or for those using numerical methods in analyzing problems of this type. A correlation for average convective Nusselt number is also provided. (C) 2001 Elsevier Science Ltd. All rights reserved.