This paper discusses several ways of improving thermodynamic efficiency in membrane separation other than pressure ratio optimization, such as optimal process flow-sheet design and membrane selection, and explains the relationship between the various factors affected by these measures and thermodynamic efficiency. It also describes the relationship between stage and separator thermodynamic efficiency and local thermodynamic efficiency and permeation rates. It discusses the definition of the extent of separation and stage and local separation ability and explains why, when the type of membrane and the pressures on the two sides of a membrane are fixed, the more thermodynamically efficient membrane-separation process typically also needs less membrane area for separation tasks with specified purities for both permeate and retentate. It then briefly discusses cases with mixing of streams with different compositions and the objective function for optimal design of membrane separators, as well as the effects of back-diffusion, mass-transfer resistances, and cross flow.