The magnetic moment of the half-metallic ferromagnetic oxide CrO2 has been modified via electrochemical lithiation. In the low-Li regime, below about x = 0.2 in LixCrO2, a large change in magnetization of 20 2% per 0.1 Li/formula unit is obtained, in reasonable agreement with a model based on the change in oxidation state of the Cr ions. The change in magnetization is partly reversible, especially for x 0.1. The kinetics and reversibility of the process are improved at 60°C, and Li contents up to x = 0.9 could be inserted. At these concentrations the room-temperature moment is reduced to 5% of its initial value. This decrease is irreversible, due to disruption of the rutile crystal structure of the CrO2.