A method and arrangement are proposed for injecting CO2 into a subterranean aquifer for storage therein. In order to reduce the effects of water evaporation from brine in the aquifer when dry CO2 is injected into the aquifer, the CO2 is supplied mixed with a salt-lean fluid, i.e. a fluid that contains a low concentration of ions that can precipitate out as salts. The mixing may take place at the wellhead, with the CO2 salt-lean fluids supplied via separate low-grade material pipelines. The proportion of CO2 and salt-lean fluid in the mixture is such as to obtain a CO2 composition that is saturated with salt-lean fluid at the site of injection into the aquifer. By injecting saturated or “wet” CO2, less water is evaporated from the brine and the salt precipitation is greatly reduced, so keeping the pore spaces clear and providing an increased accessible pore volume for CO2 storage.