A naphthalene sublimation technique is employed to investigate the convective mass transfer process from a square cylinder and its base plate in a flow of air. Distributions of local mass transfer coefficients on each face of the cylinder change dramatically with the angle of attack. The average Sherwood number has a minimum value at α = 12-13°, and a maximum value at α=20-25° where a is the angle of attack. A comparison of the present mass transfer measurement with earlier heat transfer measurements, using the heat/mass transfer analogy, shows good agreement at average transfer rates, but notable differences in local values, especially near the reattachment point. A remarkable enhancement of mass transfer due to the horseshoe vortex system is observed on the square cylinder and on the base plate near the plate-cylinder junction. As the angle of attack increases, the mass transfer peak created by counterrotating vortices diminishes. On the other hand, the influence of the corner vortex remains strong regardless of the angle of attack. Surface flow on the base plate is visualized using the oil-lampblack technique. Streaks of the corner vortex and the counterrotating vortices in the visualization picture compare well with the trails of the peak of mass transfer created by these vortices.