In this study, pre-breakdown phenomena in CO2, air, and air/CO2 mixtures are characterized under positive voltage impulses. A point-plane geometry (tip radius of curvature: 0.2mm, inter-electrode distance: 50mm) is used to generate a divergent electric field and facilitate discharge inception. Breakdown voltage measurements and time-resolved discharge characterization are performed for pressure between 0.1 MPa and 0.5 MPa. In air, breakdown results from streamer propagation up to 0.4 MPa, followed by a transition to the leader mode for higher pressures. CO2 shows a similar transition above 0.2 MPa, and the mixtures have and intermediate behavior.