This paper examines the role of prior knowledge in offsetting CO2 emissions from fuel consumption in vehicle use. The main result of our econometric analysis on the basis of unique representative data from drivers' license holders in the USA and Germany refers to the strong discrepancy between the stated and revealed knowledge of CO2 offsetting. The revealed knowledge - measured by the correct estimation of the costs for this offsetting practice - has a positive impact on the stated purchase of corresponding offsetting credits in both countries. In contrast, the effect of the stated knowledge of offsetting is not robust and in some cases even negative. Surprisingly, it cannot even be confirmed that drivers' license holders who claim that they are more than sufficiently informed estimate these costs more accurately. (C) 2011 Elsevier Ltd. All rights reserved.