Depleted hydrocarbon reservoirs are key targets for geological storage of CO2. It is well known that Joule-Thomson cooling can potentially occur in reservoirs during CO2 injection. In this paper we investigate the impact of the presence of other gases (impurities) in the injected CO2 stream on Joule-Thomson cooling. A coupled heat and mass transport model is presented that accurately accounts for the pressure-, temperature-, and gas-compositional influences on the thermo-physical transport properties such as density, viscosity, specific heat capacity and Joule-Thomson coefficient. With this model it is shown that impurities affect both the spatial extent of the zone around the well bore in which Joule-Thomson cooling is induced and the magnitude of the cooling. SO2 expands the zone of cooling, O2, N2, and CH4 contract this zone, and H2S has a very small influence on the spatial extent of cooling. These relative behaviours are primarily controlled by the impact of the impurities on the specific heat capacity of the gas mixtures.The influence of impurities on the magnitude of cooling also depends on the operational conditions of gas injection. Enhanced cooling is caused by O2, N2, and CH4 in combination with constant pressure injection, while for constant rate injection cooling enhancement is minimal or absent. Presence of SO2 strongly suppresses Joule-Thomson cooling at low injection temperatures. Apart from the Joule-Thomson coefficient, the density of the gas mixture plays an important role in controlling these thermal responses.The thermal risks associated with impure gas injection appear small. Enhanced cooling >5K requires high-pressure, low-temperature injection in a low permeability reservoir and presence of O2, N2, and/or CH4 in the injectate. Co-injection of SO2 has clear beneficial thermal consequences for low-temperature injection, by suppressing Joule-Thomson cooling, and may therefore be of special interest to help bring down the costs of CO2 sequestration in depleted gas reservoirs. © 2013 Elsevier Ltd.