The effect of a mixing vane was studied in a three-subchannel geometry for a 3.99-mm (0.157-in.) gap space, and for mass velocities of 339.0, 678.1, and 1356.2 kg/s⋅m2 (0.25, 0.50, and 1.0 Mlb/h⋅ft2, respectively). Mixing rates increased rapidly downstream of the vane and then decreased gradually as a function of downstream distance. The maximum increase in turbulent cross-flow for a mass velocity of 339.0 kg/s⋅m2 occurred ∼10 hydraulic diameters from the end of the mixing vane and was 150% greater than the value observed at comparable conditions for the same test section without a mixing vane. The average increase in mixing due to the presence of the vane was ∼58%.