Abstract CO2 geo-storage in basaltic formations has recently been demonstrated as a viable solution to rapidly sequester and mineralize CO2. In case CO2 is injected into such basalt reservoirs in supercritical form, a two-phase system (reservoir brine and supercritical CO2) is created, and it is of key importance to specify the associated CO2-basalt wettability so that fluid distributions and CO2 flow through the reservoir can be predicted. However, there is a serious lack of data for basalt CO2-wettability. We therefore measured water contact angles on basalt substrates in CO2 atmosphere. The results indicate that at shallow depth (below 500 m) basalt is strongly water-wet. With increasing depth the basalt becomes less hydrophilic, and turns intermediate-wet at a depth of 900 m. We conclude that basalt is more CO2-wet than chemically clean minerals (quartz, calcite), especially at depths below 900 m. However, the basalt had a CO2-wettability similar to some caprock samples and a gas-reservoir sandstone. The data presented in this paper will thus aid in the prediction and optimization of CO2 geo-storage in basalt formations.