This is the third of three related papers presented in this volume on mixed convection heat transfer in vertical passages. In it an experimental study is reported of buoyancy-influenced convective heat transfer to air flowing upwards and downwards under turbulent conditions through a vertical plane passage of relatively short length with a uniform heat input on one wall and an adiabatic condition on the opposite one. Thermal development and flow development occurred simultaneously and neither were complete. As in the case of the studies reported in the two preceding papers, the results presented here reinforce the existing picture of buoyancy-aided mixed convection in vertical passages, which hitherto has been based largely on experiments performed with uniformly heated circular tubes. However, the effects of buoyancy were slightly weaker than for circular tubes, onset of buoyancy-induced impairment of heat transfer was delayed and it occured more gradually, These trends are consistent with the findings reported in the two preceding papers and can again be explained by the fact that only one surface was heated and, therefore, turbulence was only modified directly by buoyancy in one of the two boundary layers.