Following scandals about corruption in foreign aid, and in a political climate that increasingly questions the legitimacy of development assistance, donors are under pressure to better control how their funds are spent. However, there is little evidence on precisely how to control corruption in development aid. This article assesses under which conditions donor regulations are successful in controlling corruption in aid spent by national governments through procurement tenders. The article analyses data on donor-funded procurement contracts in 100+ countries in 1998–2008 and uses ‘single bid submitted in a competitive tender’ as a corruption risk indicator. Applying a contract-level propensity score matching and regression analysis, it finds that an intervention which increases donor oversight and widens access to tenders is effective in reducing corruption risks: lowering single bidding on competitive markets by 3.6–4.3 percentage points. This effect is greater in countries with low-state capacity.