The in vitro aerosol performance of two combination dry powder inhaler (DPI) products, Foster® NEXThaler® and Seretide® Diskus® were investigated with single particle aerosol mass spectrometry (SPAMS). The in-vitro pharmaceutical performance is markedly different for both inhalers. Foster® NEXThaler® generates a higher fine particle fraction (FPF <5 μm) and a much higher relative extra fine particle fraction (eFPF <2 μm).