This paper examines the valuation implications of greenhouse gas (GHG) emissions allowances. We posit that the value of a firm's bank of emission allowances has two components that are likely to be positively valued by the capital market: (1) an asset value component; and (2) a real option value component. Since the necessary data to examine this research hypothesis in the setting of GHG emission allowances is not yet available, we test our conjecture by examining the value relevance of sulfur dioxide (SO2) emission allowances held by US electric utilities. Empirical results reveal that the capital market assigns a positive price to a firm's bank of SO2 emission allowances, consistent with the argument that emission allowances have, at least, an asset value component that is assigned a positive price by the market. We also find weak evidence consistent with the market assigning a real option value to the allowance banks.