A study of the near-wake flow of castellated blunt-trailing-edge aerofoils at a Mach number of 2 was conducted to understand the nature of the flow and the mechanisms of base pressure recovery. The investigation has shown that strong gradients exist in the spanwise direction and that the formation of the wake recompression shock occurs further away from the wake axis. Also, the wake neck is broader and diffused. Detailed quantitative data involving pressure measurements, schlieren and holographic interferometry, and laser transit velocimetry, are presented. A theoretical model to predict the mean base pressure on a castellated base is also proposed. Comparison with experimental data shows that the model provides a qualitative description of the flow behind a castellated base at supersonic speeds.