The mass transfer from a square cylinder and from the plate on which the cylinder is mounted vertically, is investigated with the naphthalene sublimation technique. The general pattern of local mass transfer is somewhat different from that with a circular cylinder. A comparison with heat transfer measurements on a square cylinder in the two-dimensional flow region, using the heat/mass transfer analogy, shows good agreement in average transfer rates, but slight differences in local values. A dramatic change of mass transfer rates is found both on the cylinder and the base plate around the plate-cylinder junction region due to a horseshoe vortex system. Multiple vortices, which include the primary horseshoe vortex, the corner vortex and two pairs of counterrotating vortices, affect the mass transfer process. Variation of either Reynolds number or initial boundary layer thickness does not significantly change the location of the peaks created by the horseshoe vortex system, but only affects the magnitude of the local mass transfer rate. Visualization of the surface flow on the base plate is included to supplement the mass transfer measurements.