This study is a comparative analysis of the development of energy intensity and CO2 emission intensity in the ASEAN countries in the years 1971-97. The reasons and driving forces behind the changes are studied in the article using a complete decomposition methodology. The decomposition analysis explains energy use or CO2 emissions with three different factors: activity effect. structural effect and intensity effect. The authors provide an analysis of dynamic changes of energy consumption and CO2 emission flows. Intensity effects on energy use and CO2 emissions revealed large differences in the performance of ASEAN countries. An interesting finding of the study is that in the 1990s the intensity effects on CO2 emissions in ASEAN countries have started to increase indicating decreasing CO2 efficiencies of the economics. The general observation on the basis of decomposition analyses is that the phase of industrialization is increasing the energy intensity of the nations and. when the increased energy demand is fulfilled with fossil fuels. the CO2 emissions will inevitable grow. However. it has to be recognized that the CO2 emissions per capita in ASEAN countries are very low (except in Singapore) compared with many OECD countries.