The rheological properties of drilling fluids are crucial parameters in offshore operations, especially in the extreme conditions encountered during deepwater drilling. Oil based muds (OBM) are almost exclusively used in deepwater drilling operations, owing to such factors as their improved temperature stability, effectiveness when drilling through water sensitive formations, and capability to be utilized in narrow-margin drilling. A desired property of the drilling fluid is a minimal sensitivity of the flow properties with respect to temperature, leading to a flat-rheology system. Chemical additives provide the basis for the fluid's rheological properties, but are rarely addressed in detail within published literature. This paper reviews the chemistries of existing additives and their claimed uses. In addition to addressing the current state of the art, results are presented from an investigation into the effects of such emulsifier chemistries upon the rheological properties of the drilling fluid. This includes work on a novel emulsifier and comparative data to an industry standard incumbent. Through an improved study of the chemistry, a design-based approach can lead to the development of optimal properties for these high performance invert emulsion drilling fluids.