Many energy-intensive process industries have complex material flows, which have a strong effect on the overall energy intensity of the final product (OEIF). This problem, however, has only been recognised qualitatively due to the lack of quantitative analysis methods. This paper presents an in-depth quantitative analysis of the effect of material flows on energy intensity in process industries. Based on the concept of a standard material flow diagram (SMFD), as used in steel manufacturing, the SMFD for a generic process industry was first developed. Then material flow scenarios were addressed in a practical material flow diagram (PMFD) using the characteristics of practical process industries. The effect of each material flow deviating from a SMFD on the OEIF was analysed. The steps involved in analysing the effect of material flows in a PMFD on its energy intensity are also discussed in detail. Finally, using 1999 statistical data from the Chinese Zhenzhou alumina refinery plant, the PMFD and SMFD for this plant were constructed as a case study. The effect of material flows on the overall energy intensity of alumina (OEIA) was thus analysed quantitatively. To decrease OEIA, the process variations which decrease the product ratios could be employed in all except in multi-supplied fraction cases. In these cases, the fractions from the stream with lower energy intensities should be increased. (c) 2005 Elsevier Ltd. All rights reserved.