The simultaneous effects of diffusion and coherent averaging by magic angle sample spinning (MASS) are investigated both theoretically and experimentally for spins diffusing in a spatially varying susceptibility field. The diffusion introduces a periodic modulation and a relaxation of the magnetization amplitude. The attenuation exponent for the nth echo at an early time is given by D(0)gamma (2)g(2)pin(4 omega (3)(r)), where omega (r) is the rotor frequency, D-0 the diffusion coefficient, gamma is the gyromagnetic factor, and a factor g(2) which is related to, but different from, the volume average of the gradient squared.