Results are reported of an experimental investigation in the thermal entrance region of a horizontal parallel-plate channel when the lower plate is heated and the upper one is cooled. The experiments covered a range of Rayleigh numbers between 103 and 3.1 × 104 and Reynolds numbers between 30 and 1100 using air. Measurements of Nusselt numbers and temperature distributions indicate much higher critical Rayleigh numbers than the theoretically predicted values. Beyond critical Rayleigh numbers second-type vortex rolls are predominant and the local Nusselt number increases gradually with the Rayleigh number. The thermal-entrance length as determined from temperature profiles does not show appreciable change with the Rayleigh number.