Velocity characteristics of the flow in a round cross-sectional S-shaped diffusing duct with an offset of 0.3 of the length (axial) of the duct were measured, using a laser Doppler anemometry and refractive index matching technique, with two inlet boundary layer thicknesses, corresponding to 0.5 and 0.2 of the inlet duct diameter at a fixed Reynolds number of 4 x 10 s. The duct had inlet and exit diameters of 48 mm and 56.8 mm respectively, a length of 170.4 mm and an offset between the centre-line of the inlet and exit of 51.1 mm and the dimensions corresponded to a scaled model of the Royal Aerospace Establishment 2129 duct used for jet aircraft intakes. The previously speculated separation regions were confirmed and found to be larger with the thinner inlet boundary layer and, as a consequence, the exit mean flow distribution consisted of a large pair of contra-rotating vortices occupying almost half of the crosssection with maximum secondary mean velocity 20% of the inlet bulk velocity, compared to a third of the cross-section in the thicker inlet boundary layer and with maximum secondary mean velocity 15% of the inlet bulk velocity.