  We study the uncertainties of the Standard Model (SM) predictions for CP
violating B decays and investigate where and how supersymmetric (SUSY)
contributions may be disentangled. The first task is accomplished by letting
the relevant matrix elements of the effective Hamiltonian vary within certain
ranges. The SUSY analysis makes use of a formalism which allows to obtain
model-independent results. We show that in some cases it is possible a) to
measure the CP B-\bar{B} mixing phase and b) to discriminate the SM and SUSY
contributions to the CP decay phases. The golden-plated decays to this purpose
are the B \to \phi K_S and B \to K_S \pi^0 channels.
