  We construct a large class of SU(5) orientifold vacua with tadpole
cancellation both for the standard and the flipped case. We give a general
analysis of superpotential couplings up to quartic order in orientifold vacua
and identify the properties of needed Yukawa couplings as well as the baryon
number violating couplings. We point out that successful generation of the
perturbatively forbidden Yukawa couplings entails a generically disastrous rate
for proton decay from an associated quartic term in the superpotential,
generated from the same instanton effects. We search for the appropriate
instanton effects that generate the missing Yukawa couplings in the SU(5) vacua
we constructed and find them in a small subset of them.
