International investment law has received increasing attention due to the proliferation of investment treaties and the number of arbitral awards made thereunder. At the end of 2011, there were 450 known investment cases that have been settled or are pending. Yet, there are many core questions that remain to be authoritatively answered. Although they are faced with similar problems, arbitral tribunals often adopt diverging solutions to investment disputes. This paper considers the nature of the divergences in investment treaty jurisprudence and the role that could potentially be played by the International Law Commission (ILC) in contributing to the coherent development of international investment law. The paper argues that some areas of international investment law are more appropriate for attention by the ILC than others. It draws a distinction between those aspects of international investment that only have a basis in treaty law and those aspects of international investment law that are underpinned by common standards stemming from customary international law or general principles of law. The paper argues that we cannot necessarily expect the convergence of jurisprudence in the context of treaty provisions that have been specifically negotiated by the parties, as these provisions must be interpreted on a case-by-case basis. This means that topics like the MFN clause are less suitable for codification, as the meaning of these provisions will often depend on the particular context of the treaty and the precise intentions of the parties. In contrast, there is a stronger case for the codification of international investment law where common standards exist. The paper therefore considers the formation and development of customary international law in relation to investment protection. It argues that whilst investment treaty tribunals have struggled with the identification of customary international law in this area, the ILC could play a central role in clarifying the state of the relevant rules and principles, in furtherance of its core mandate of promoting the progressive development and codification of international law.