Uncertainty makes objectives harder to reach. This article examines whether uncertainty in subsidies leads to mission drift in microfinance institutions (MFIs). Using a worldwide sample of 1,151 MFIs active in 104 countries, we find that interest rates increase with aid volatility while average loan size (ALS) is inversely related to aid volatility. These results suggest that MFIs consider ALS as a signaling device for commitment to their social mission, but use interest rates as an adjustment variable to cope with uncertainty. The policy prescription to donor agencies wishing to curtail the rise in interest rates is to deliver subsidies predictably and transparently.