The present project is on a new software environment for thermodynamic considerations based on object-oriented programming styles. At its heart is a graphical modeling language for interactive definition of thermodynamic problems. The modeling language is based on seven modeling elements that are linked to express cause-acid-effect relationships. The language can generate problem definitions ("models") graphically (i) that capture the user’s ideas and knowledge, (ii) that are thermodynamically consistent and (iii) that can be used for various purposes such as thermodynamic property calculations, detailed-analysis of mixture behaviour (phase stability, ...) and selection of appropriate calculation methods.