We performed boiling flow experiments and measured the void fraction in a 3 × 3 rod bundle including a spacer grid with split type vanes using X-ray computed tomography, which provides high-resolution time-averaged void data without disturbing the flow. We studied the effects of mixing vanes with different vane angles, namely, 20°, 29° and 40°, for a mass flux between 535 and 1950 kg/m² s and the central rod being heated giving a heat flux of 85.7 kW/m². The experiments were conducted using octafluorocyclobutane (RC318) as the working fluid. The presence of vanes leads to an increase of the cross-sectional averaged void fraction up to an axial position of Z≈0.8D_h. After that, the void fraction decreases until 3D_h<Z<4D_h due to the induced swirl flow, before it increases again. It was further found that the vanes cause a high local void fraction near the spacer, which increases the possibility of DNB occurrence. From this study, it can be concluded that a vane angle of 29° is optimal for two-phase flow, which is consistent with the findings in the literature for single-phase flow.