Transport of dense phase CO2 by pipelines needs to account for possible degradation mechanisms arising due to H2 impurity which can be present in (typically) the 0.75 ~ 1 mol% range depending on the specification used. H2 gas can affect fatigue and fracture properties even at low partial pressures and hence the integrity. In this study a complementary experimental and computational modeling program was undertaken. API 5L X65 grade material samples were used for comparable testing in pure H2 gas at low partial pressure H2 and in dense phase CO2 at 100 barg with H2 impurity added in a 2 mol% and 4 mol% increments (i.e. partial pressures of 2 barg and 4 barg). The test results show effect of the H2 impurity in the dense phase CO2 accelerated fatigue crack growth and reduced fracture toughness. These findings are supported by computational simulation using density functional theory and molecular dynamics. A scoping integrity assessment for a dense phase CO2 pipeline with H2 impurity at 2 mol% shows the measured accelerated fatigue crack growth only affects large pressure cycles (pressure range in excess of 70 bar for OD/WT pipelines of ~30). The fatigue life is significantly shortened compared to the pure CO2 pipeline and this is mainly driven by the reduced fracture toughness at low partial pressure H2. The fatigue damage is proportional to the maximum operating pressure (as it increases the pressure range). For a postulated 50-year design life a safe pipeline design window considering a range of conservative inputs (material properties, integrity operating window, geometry, postulated defect size etc.) a safe operating pressure of 2900 psig (200barg) was found for a new build pipeline using modern materials. For a repurposed vintage material pipeline the safe operating pressure was set at 2200 psig (150 barg). An ECA incorporating the deleterious effect of H2 should be conducted to confirm the above postulate unless the project falls within the limits of the conservative inputs (material properties, integrity operating window, geometry, postulated defect size etc.), from a fracture mechanics perspective, of the four case studies that form the basis of this overall study. Other impurities if exceeded their safe threshold may create water drop out and acid formation. It is imperative that the project specifications err on the side of caution and restrict the impurities to safe limits until further understanding of the complex interaction mechanisms is developed.