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Cadmium telluride (CdTe) solar cells have achieved efficiencies of over 22%, despite having absorber layer grain sizes less than 10 μm and hence a very high density of grain boundaries. Recent research has shown that this is possible because of partial passivation of grain boundaries during the widely used cadmium chloride treatment, and passivation of grain interior defects by selenium alloying of the CdTe. Here, state-of-the art TEM-based cathodoluminescence imaging is used to show that, in addition to grain interiors, selenium also passivates grain boundaries in alloyed Cd(Sex,Te1-x) material (CST). Specifically, we find that recombination at CST grain boundaries is up to an order of magnitude lower than at CdTe grain boundaries. This further explains the superior performance of selenium graded CdTe devices and provides potential new routes for further efficiency improvement and solar electricity cost reduction.