The quantifying and reporting of greenhouse gas emissions is one of the most important tools for monitoring and auditing proposed to mitigate climate change, and it also directly affects business. It is thus vital that at this time we learn in detail whether firms actually report on greenhouse gas emissions and make the account entries that must be included within it. This research is twofold: first to analyse the reports on greenhouse gas emissions of international firms in the 2007 and 2008 period and to see what kind of variation occurs in CO2 emissions between 2006-2007 and 2007-2008, and second to determine the impact that this variation (2006-2007) can have on firm performance in four time periods (t, t + 1, t + 2 and t + 3) that correspond to 2007, 2008, 2009 and 2010, taking two variables as a measure of firm performance, ROE and ROA, and considering a time period affected by a financial crisis. The results obtained show that there was a reduction in CO2 emissions in the 2006-2007 period, and also in the 2007-2008 period. As regards the impact that the variation in CO2 emissions has on ROE and ROA, CO2 emission variation is a significant but negative variable only for ROA_2007 and for the rest of the years it is not statistically significant either for ROE or ROA. Copyright (c) 2012 John Wiley & Sons, Ltd and ERP Environment.