The developing secondary flow patterns caused by buoyancy forces in the simultaneous hydrodynamic and thermal entrance region of a horizontal rectangular channel (aspect ratio, a/b = 2) with isothermally heated lower wall and/or isothermally cooled upper wall are studied using the smoke injection method. The entrance air is at room temperature (around 23 degrees C), and the temperature ranges for the lower and upper walls are 23-51 degrees C and 15-23 degrees C, respectively. The mean air velocities are 0.1, 0.2, 0.3, and 0.4 m/s. The Reynolds number varies from Re = 2.10 X 10(2) to 1.05 X 10(3) and the Grashof number ranges from Gr = 1.56 X 10(5) to 5.04 X 10(5). Photographic results reveal mixed convection, convective instability, and chaotic phenomena. Many new secondary flow patterns are revealed by this flow visualization study, and the photographic results should be useful for future theoretical study and measurements. The experiments were designed to understand flow physics.