At the request of several of its members PRCI initiated a JIP to address a critical research need for CO2 transmission pipeline design and analysis. The objective is to determine experimentally the decompression wave speed in predominantly CO2 mixtures with N2, O2, CO and H2 impurities, from a range of initial pressures and temperatures. Such impurities are typical of carbon capture streams from industrial sites that must be transported by pipeline for sequestration or enhanced oil recovery (EOR) projects. Decompression wave speed is an essential element for achieving effective fracture control design, which is required for all pipelines transporting hazardous substances. The final goal is to provide essential information to allow CO2 pipelines to be designed with appropriate material toughness to prevent long running ductile fractures, thus removing a potentially serious obstacle to their efficient design for high-consequence areas. Another technical objective is to compare the test results with various equations of state and determine which ones best represent the properties of both pure CO2 and CO2 with impurities, particularly in the dense or supercritical state.