Presented herein are boundary-layer studies of the steady mass-transfer phenomena in an aligned-field magnetohydrodynamic flow past a semi-infinite flat plate. The fluid is assumed to be viscous, electrically conducting and incompressible. The plate either is porous such that blowing or suction of a foreign diffusing fluid through the wall surface can be effected, or is made of a material which sublimes into the hot boundary-layer fluid, thus inducing a mass-transfer process. Other parameters being held constant, the skin friction, heat-transfer (except for negative Eckert numbers) and mass-transfer coefficients are decreased by a convective flow away from the wall surface (e.g. blowing or sublimation) and are increased by a convective flow toward the surface (e.g. suction or icing). The effects of the applied magnetic field are such that, for blowing and small suction rates, the magnetic field decreases the skin friction, the induced magnetic field at the wall, the local heat-transfer coefficient and the local masstransfer coefficient; but at higher suction rates, it increases all the above parameters, except for negative Eckert numbers. © 1968.