Market instruments are a core element of policies to mitigate climate change. Choosing and designing the right instruments can help to minimise mitigation costs. This paper seeks to analyse various market instruments for CO2 mitigation from the viewpoint of cost-effectiveness, using an AGE (applied general equilibrium) model for the case of Spain. A distinction is drawn between (1) quantity instruments, which represent different extents of a market for emission permits; and (2) price instruments, which represent different types of tax. The analysis quantifies the importance of making the right choice as to which sectors are excluded from the permit market, and of targeting taxes correctly.