The transport sector has an important and increasing role in greenhouse gases emissions, and cost benefit analysis (CBA) of transport projects should give in this regard accurate and objective information. Indeed, many countries have included this concern in their CBA guidelines, but it typically consists simply in adopting an official value per ton of carbon emitted. Does this mean that the issue is correctly treated by CBA? Since “the devil is in the details” this paper reviews key items influencing the quality of CO2 emission valuation, estimating the order of magnitude of their effects and taking into account the nature and degree of imprecision and uncertainty associated with them.