We show that, given a weak
compactness condition which is always satisfied when the underlying space does not
contain an isomorphic copy of c0, all the operators in the weakly closed algebra
generated by the real and imaginary parts of a family of commuting scalar-type
spectral operators on a Banach space will again be scalar-type spectral operators,
provided that (and this is a necessary condition with even only two operators) the
Boolean algebra of projections generated by their resolutions of the identity is
uniformly bounded.