The dynamics of carboxyl-modified polystyrene colloidal particles of varying size and zeta potential were investigated in a microfluidic junction device where merging electrolyte streams generated a steady-state salt concentration gradient. The device featured a long microchannel downstream of the junction, enabling the particle dynamics to be analysed over a time scale comparable to the relaxation time of the salt gradient. In the bulk of the channel, particles migrate outwards through a diffusive-like process arising from the additive contributions of particle diffusiophoresis and wall diffusioosmosis. An analytical expression is derived for the effective diffusion coefficient, and a microfluidic approach is proposed to estimate particle diffusiophoretic mobility. Particles accumulated at the walls also displace sideways but the direction of their motion depends on the counter-acting effects of particle diffusiophoresis and wall diffusioosmosis. This motion is sub-diffusive at short distances from the junction, but either ceases or reverses further downstream. The intensity of the accumulation peaks exhibits a non-monotonic dependence on the distance from the junction and, after calibration, can serve for the microfluidic measurement of particle size or zeta potential, when one property is known a priori. The influence of electric double layer polarisation on diffusiophoretic mobility is also examined.