Mass transfer to the bottom wall of a rectangular cavity in the presence of an external periodic flow is investigated. Flow unsteadiness is generated by placing a circular cylinder at mid height within a channel. The cavity is located downstream of the cylinder at the lower wall of the channel. Both experimental measurements and numerical simulations are carried out to establish the ability of a time-accurate finite element solver to capture the transient flow and mass transfer. The mass-transfer rate at the bottom wall of the cavity is measured using reduction of ferricyanide ions on a platinum line electrode. Cavity aspect ratios (height/width) of 0.5, 1.0 and 1.81 and Reynolds numbers based on channel heights between 100 and 800 are investigated.