Local heat transfer and flow behavior are investigated for laminar and transitional flows in sinusoidal wavy passages. The experimental geometry consists of a channel with a 10: 1 aspect ratio bound by two wavy walls. The walls are from 12 to 14 wavelengths long, and the wave amplitude, phase angle, and wall-to-wall spacing are varied during the experiments. Using visualization methods, the flow field is characterized as steady or unsteady, with special attention directed toward detecting the onset of macroscopic mixing in the flow. The location of the onset of mixing is found to depend on the Reynolds number and channel geometry. Instabilities are manifest near the channel exit at low Reynolds numbers (Re similar to 200) and move toward the channel entrance as the Reynolds number is increased; the entire channel exhibits unsteady, macroscopic mixing at moderate Reynolds numbers (Re similar to 800). The onset of macroscopic mixing is directly linked to significant increases in local heat transfer.