This paper describes sparkover characteristics of CF3I-CO2 gas mixture, with focus being given on 30%-70% ratio. Standard lightning impulse of 1.2/50 is used in this study, along with three different electrode configurations with variable gap lengths. It is found that higher gap length provides higher U50 and Emax for a given electrode configuration, although the rise in U50 is more obvious in sphere-sphere configuration, due to more uniform electric field. Breakdown strength is also increased with gas pressure, with tests being carried out up to 2 bar (abs).