This study analyzes the determinants of banks' net interest margins during 2008-2014, when monetary policy measures were expansionary. The authors estimate a model where net interest margin depends on factors including: short-term interest rates; the slope of the yield curve; market power; credit risk; interest rate risk; costs; and reserves. The results suggest net interest margins are positively affected by short-term rates and the yield curve slope, but the relationships are nonlinear. Credit risk also positively impacts margins, while costs, liquid reserves, and efficiency negatively affect margins.