A generalized integral transform technique is used to obtain various order analytical solutions to the problems of linear diffusion of radioactive atoms in an eroding slab, cylinder, or sphere subjected to a general boundary condition of the third kind. The zeroth- and first-order solutions obtained in this manner are compared with the exact solution; the results are highly accurate for most practical cases. The proposed technique is sufficiently versatile to cover a variety of heat and mass transfer problems in media with moving boundaries.