In the past, many studies on energy efficiency levels were not comparable due to differences in economic structure between countries. In the project ＇International Comparisons of Energy Efficiency＇ efforts are undertaken to develop methods that do account for such differences. In this paper, we identify structural differences in energy intensive industries and describe ways to incorporate these differences in international comparisons of energy efficiency. For the iron and steel, aluminium, cement, pulp and paper, ammonia, chlorine and alkali, and petrochemicals sectors, structural differences mainly arise in product (quality) mix and import/export streams. In addition to structural indicators, also non-structural, explanatory indicators are identified, such as the penetration of energy efficient equipment and Combined Heat and Power generation. Feedstock mix and process type can either be structural or explanatory indicators, depending on whether or not product mix is affected. A number of issues regarding data quality and other pitfalls are described, mainly related to aggregation level and system boundaries between different industry sectors, and between the industry and energy transformation sectors. The methodologies developed show that structural differences can be taken into account in cross-country comparisons of energy efficiency if appropriate physical energy efficiency indicators are used.