This exploratory study analyses voluntary disclosures of greenhouse gas (GHG) emissions to the Carbon Disclosure Project (CDP) by the largest emitters in the US utilities sector. We compare the amounts that firms voluntarily disclose to the CDP to mandatory amounts that should be less than them and find that the opposite is true for some firms. Examining the comparability of the CDP amounts, we find that firms calculate them using different reporting boundaries and accounting methodologies. We analyse how the choice of boundary affects emissions and find that it results in differences in emissions amounts. We also find that most firms do not have their reported amounts verified by third parties. Our analysis indicates that the amounts that some of the largest emitters disclose to the CDP are unreliable and incomparable. Our results suggest caution in relying on voluntary GHG emissions disclosures.