  We study in detail the d=6 operators for proton decay in the two possible
matter unification scenarios based on SU(5) gauge symmetry. We investigate the
way to distinguish between these two scenarios. The dependence of the branching
ratios for the two body decays on the fermion mixing is presented in both
cases. We point out the possibility to make a clear test of flipped SU(5)
through the decay channel p \to \pi^+ \bar{\nu}, and the ratio \tau(p \to K^0
e^+_{\alpha}) / \tau(p \to \pi^0 e^+_{\alpha}).
