We use the perturbative QCD methods of Lepage and Brodsky to calculate the rate for \bar B_s\rightarrow \rho K_S , with an eye toward the CP violating unitarity triangle angle \gamma . We show that, although the penguins are large, there are regions of the allowed parameter space of the Cabibbo--Kobayashi--Maskawa (CKM) mixing matrix wherein \gamma is measurable in the sense that penguins change of the value of \mathop sin (2\gamma ) one would extract from the attendant time dependent asymmetry measurement by less than 29%, so that a 3\sigma measurement of sin (2\gamma ) as being different from 0 is allowed by the corresponding theoretical uncertainty. This would establish CP violation in B_s decays. The rates which we find tend to favour the type of luminosities now envisioned for hadron-based B-factories.

PACS numbers: 12.15.Hh, 13.25.Hw, 12.38.Bx, 14.40.Nd