Positive displacement motors have been used extensively in underbalanced drilling operations. Motor performance characteristics are essential to ensure drilling success and efficiency. Pressure drop across the motor must be understood to accurately simulate underbalanced conditions bottom-hole. In this study, five positive displacement motors were tested under a variety of mixing ratios of nitrogen and water with different back pressures. Comparing with the baseline test at maximum liquid flow rate, the replacement of water with nitrogen decreased the motor performance by as much as 95% in terms of maximum torque output and maximum mechanical power output. The pressure drop across the motor was lower with commingled fluid, and the greater the nitrogen ratio, the lower the pressure drop across the motor. It was also found that back pressure decreased the motor performance. This paper also discusses the testing procedures, the results, and how commingled fluid affects underbalanced drilling operations.