This article recommends that the likely legal nature of carbon units should be considered early on in the design process for an emission trading system. A review of common law cases dealing with a variety of statutory instruments is used to illustrate the key principles courts will use in assessing whether carbon units are a form of property. Consideration is then given to how the general principles that arise from these cases can be applied in the design of legal regimes involving tradable carbon units. Pro-active consideration of the legal consequences of carbon units being treated as property can provide long term benefits.