Orbal multi-channel oxidation ditches have received relatively little attention and thus knowledge of their characteristics is not as highly advanced as for other oxidation ditch systems. Dye tracer and dissolved oxygen measurements have been undertaken to elucidate the mixing characteristics of a three-channel Orbal system treating 80 Ml/d. The dissolved oxygen concentrations showed a complex variation in space due to the input, approximating a continuous line source, non-uniform distribution of turbulence and secondary flows caused by channel configuration. Consideration of the DO measurements and the dye tracing results suggest that the flow could be treated as 2 CSTRs in the outer channel and a single CSTR in each of the two inner channels.