As part of Carbon Capture and Storage (CCS) projects, Carbon Dioxide (CO2) is usually transported via pipelines from source to sequestration location. Ensuring the safety of the operation is of great importance, as CO2 is a hazardous substance and an accidental release may have catastrophic consequences. Therefore, a comprehensive understanding of the effects of a CO2 release from CCS facilities is essential to allow the appropriate safety precautions to be taken. The majority of prior studies that address this topic do so by simulating CO2 dispersion over a flat horizontal terrain. However, CO2 pipelines may be deployed near topographically complex locations such as congested industrial or urban areas. The extent to which the complexity of the terrain may affect the area affected by an accidental release has not been explored in detail. In this paper, we present Computational Fluid Dynamics (CFD) models for the prediction of atmospheric dispersion of CO2 over complex terrains, in order to evaluate the ‘consequence distance’ relating to accidental CO2 releases from high-pressure pipelines. The CFD model is validated against the results of a heavy gas dispersion experiment carried out at Thorney Island. Simulations of CO2 dispersion over seven types of complex terrain are carried out, considering ‘full-bore’ rupture of a pipeline carrying a pre-combustion CO2 mixture. The influence of different terrain features on the consequence distance is studied. In addition, the dispersion of a (CO2 + H2S) mixture is simulated to investigate the threshold value of the fraction of Hydrogen Sulphide (H2S) for which the hazardous effects of H2S become significant for a release over complex terrains.