The post-war hunt for known South African war criminals, as well as those who committed treasonable acts by assisting the Axis war effort, remains largely undocumented. However, this historiographical gap does not mean that South Africa did nothing to prosecute and punish those responsible for war crimes and treason. From 1945 the Union Defence Force (UDF), along with the Departments of External Affairs and Justice, made a concerted effort to determine South Africa’s exact position regarding the prosecution of wartime offences. In due course the Rein and Barrett missions were established, which collectively formed the Union War Prosecutions Section. These missions scoured post-war Europe to interview suspects and collect as much evidence as possible with the view of charging the known South African war criminals and collaborators. These investigations, one civilian and the other distinctly military in nature, ultimately met with varying degrees of success. This article specifically explores the nature, extent and effectiveness of the post-war investigations conducted by the Barrett Mission to build a treason case against Hans van Rensburg and members of the Ossewabrandwag.