The paper introduces an on-going doctoral research study aimed at facilitating the operationalisation of sustainable energy systems. The study focuses upon the analysis of existing and development of new methods for the incorporation of socioenvironmental considerations into large-scale investment decision-making in the energy sector. The final output of the research study, to be completed by the end of 2001, will be a Framework for Sustainable Energy Sector Investment Analysis and Decision Support. The Framework will be designed for application by energy companies in decision-making processes concerning the selection of new power generation alternatives. In order to broadly identify decision-making parameters applied in energy sector decision processes, primary data collection for the study involves the retrospective analysis of decision cases. The results of one portion of information gathering undertaken as part of the aforementioned data collection are presented in the paper. A retrospective case study analysis was carried out on an investment decision-making process concerning the Flintranen biomass-based district heating plant in southern Sweden. To analyse the case study da ta, an analytical module was developed. Examination of the Flintranen case using the analytical module revealed several weaknesses in the decision-making methods applied to the planning of the facility, originating primarily from the incorporation of only a limited number of decision parameters or considerations into investment calculations. Furthermore, analysis showed that, had appropriate socio-environmental decision-support methods been incorporated into the decision process, considerable financial and internal environmental costs associated with the facility could have been avoided, and environmental risk linked to its operation decreased. Lessons learned from the Flintranen case will be utilised as input into the ongoing identification and development of parameters considered as important for inclusion in the Framework for Sustainable Energy Sector Investment Analysis and Decision Support.