We simultaneously measured the flow field and wall mass transfer rate behind an orifice by means of particle image velocimetry (PIV) and the limiting diffusion current technique (LDCT), respectively. The spatio-temporal correlation coefficients of the local vertical velocity and the wall mass transfer are presented, and the canonical correlations between the proper orthogonal decomposition (POD) spatial eigenmodes in the recirculation region and the wall mass transfer rate are also discussed. A response time lag occurs between the wall mass transfer rate and the velocity field. The large canonical correlation between the POD eigenmodes and wall mass transfer rate suggests that large-scale flow structures are important to the wall mass transfer rate. In addition, the second eigenmode in the recirculation region is responsible for the peak of the mean wall mass transfer rate. (C) 2014 Elsevier Ltd. All rights reserved.