The difference in production rate between W and W− at hadron colliders is very sensitive to the the difference between upand down-quark distributions in the proton. This sensitivity allows for a variety of useful measurements. We consider the difference ds(x,Q ) − us(x,Q2) in the sea distributions and the difference ∆u(x,Q2)−∆d(x,Q2) in the polarized parton distribution functions. In both cases we construct an asymmetry to reduce systematic uncertainties. Although we discuss measurements at the Tevatron and future hadron colliders, we find that the Brookhaven Relativistic Heavy Ion Collider (RHIC) is the most appropriate hadron collider for these measurements.