In this paper we report the results of conducting a two-stage analysis on the impact and importance of mandatory adoption of international accounting reporting standards (IFRS) on European Union firms. In the first stage we determined the impact of mandatory adoption of IFRS across 13 countries and twenty industries. This was accomplished by identifying significant differences in return on assets (ROA) for firms computed under IFRS and local, generally accepted accounting principles (LG). Significant positive differences were detected for firms in Belgium, Finland, France, Italy, the Netherlands, Sweden, Switzerland and the United Kingdom: only German and Norwegian firms exhibited a negative average significant difference between ROA calculated using IFRS and LG. Repeating the analysis of differences in ROA on an industry-by-industry basis yielded additional Portuguese and Spanish firms for the second stage of the analysis in which the impact of mandatory IFRS adoption was assessed. Defining impact in terms of market and financial reporting quality, we found a statistically significant relationship between accounting information and market returns for firms in the all-countries-combined sample of 3,530 observations, and in the countries of Belgium, Finland, France, Greece, Italy, the Netherlands, Norway, Sweden and the United Kingdom. Support for the timeliness of accounting information was uncovered for firms in the all-countries-combined sample, and in the countries of Belgium, Finland, France, Germany, Italy, the Netherlands, Norway, Sweden and Switzerland. Finally, evidence to support the proposition that accounting regimes produce quality discretionary accruals was found for firms from the all-countries-combined sample of 3,480 observations and from Finland, Greece, the Netherlands, Sweden and the United Kingdom. When comparing differential accounting information constructed under IFRS and LG, however, few differences could be found. Specifically, there was no statistical support for any of the samples that accounting information produced under IFRS was any more value relevant than the accounting information derived using LG. When our examination shifted to the timeliness of earnings, a positive differential impact between earnings constructed on the basis of IFRS and local accounting standards was detected only for the all-countries-combined sample. Finally, the quality of discretionary accruals was shown to be significantly higher under IFRS than LG for firms in Finland, Greece and Sweden.