Commissioned by Carbon Market Watch and carried out by Perspectives Climate Research, this study assesses the effectiveness of carbon market grievance mechanisms of leading voluntary carbon market standards, including the American Carbon Registry, the Climate Action Reserve, Verra, Gold Standard and the Global Carbon Council. While most voluntary carbon market standards have grievance mechanisms in place, most of them are opaque and do not properly describe their procedures. Moreover, Verra charges procedural costs that are only refunded if the complaint is decided favourably. This is a clear deterrent for complaints and should be abolished. Only the Gold Standard has detailed guidance on the operationalisation of its grievance mechanism.