This paper proposes the use of Theil's second measure to analyze international energy intensity differences. This index allows differences to be broken down within and between groups of countries in a consistent manner. An analysis of OECD countries for the period 1971-1999 shows some basic points: first, the fall in energy intensities differences is attributable both to within-group and between-group inequality components; second, between-group inequalities are currently the main contributor to the whole inequality value; finally, a detailed exploration on within-group inequalities reveals the significant explanatory role played by EU-countries. (C) 2003 Elsevier Ltd. All rights reserved.