In this work we investigate the change in particle concentration near a solid boundary for colloidal dispersions in pressure driven flow, commonly referred to as wall depletion. In particular we determine the effect of Peclet number oil the strength and spatial extent of the depleted layer. The change in concentration near the solid boundary is measured with attenuated total reflection infrared (ATR-IR) spectroscopy described previously (P.J.A. Hartman Kok et al., J. Rheol. 46 (2002) 48 1). The method is capable of measuring the concentration of particles at distances ranging from 0.2 to 1.0 pin from the boundary. The Suspensions investigated consisted of mono-dispersed polystyrene particles in water. Particles of four different sizes were used, with radius, a, of 30, 54, 105, and 197 nm. (The ratio H/a was in the range 2500-17.000 with H being the height of the flow cell.) This enabled us to measure the wall depletion effect over a wide range of Peclet numbers, ranging from 0.01 to 45. We found that wall depletion was not significant for Peclet numbers smaller than unity. Estimates of the wall slip layer thickness obtained from rheological experiments were consistent with the results obtained by ATR-IR spectroscopy. (C) 2004 Elsevier Inc. All rights reserved.