In electricity markets, generators are rewarded for providing energy and for enabling grid reliability. The two functions are compensated separately: with energy market payments and ancillary services market payments. We provide evidence of changes in the generation mix in the energy market that are driven by exogenous changes in an ancillary services market. We provide a theoretical framework and quasi-experimental evidence for understanding the mechanism: it results from the multi-product nature of power plants combined with discontinuities in costs. Although much research focuses solely on the energy market, our results suggest that spillovers between the two markets are important.