Spin diffusion involves the transport of the vector magnetisation, characterised by a diffusiontensor. A magnetic field destroys the system’s isotropy so that while longitudinal diffusion is real, transverse diffusion becomes complex. The imaginary part leads to spin rotation since the spin current may no longer be parallel to the spin density. In spin polarised solid3He the molecular field of the exchange interaction leads to such an effect. We have evaluated the elements of the diffusion tensor to show this. A nonlinearity arises since the diffusion tensor so-calculated depends on the magnetisation. We show that this behaviour is described by the equation proposed by Leggett for phenomena in degenerate liquid3He. Consequently the nonlinear phenomena in liquid3He have their analogues in the spin polarised solid.