One of the tools used to optimise actions in reducing the carbon intensity of the economy is to use Marginal Abatement Cost (MAC) curves. Whilst these are known to be imperfect they are widely used to prioritise actions and often in a way that is incorrect. In particular, whilst their use for measures which have positive costs and a net reduction in carbon emissions can correctly rank the measures for effectiveness, use for ranking measures which have a net negative cost, including a wide range of energy efficiency measures, is entirely inappropriate and leads to perverse and incorrect outcomes. As a result, many assessments of low carbon actions are fundamentally flawed in their ranking of effectiveness and action is needed to overcome this and allow an optimum outcome. An alternative approach is suggested here. (C) 2014 Published by Elsevier Ltd.