The authors develop the tool of quantum diffusion (i.e. Hilbert-space-valued stochastic differential equations) for dissipative quantum systems. The aims are to find possible limitations to this approach and to investigate new pictures of open quantum systems. They are guided by the relaxation process for arbitrary spin and the associated natural rotational symmetry. They also impose the condition that the spin-coherent states remain coherent during the dissipative evolution. They present a new quantum diffusion equation that satisfies the above conditions and that is the unique quantum diffusion satisfying Percival's condition (d psi )2=0.