This paper presents optimisation technology for the computer-aided design of molecules. A new approach is presented that combines stochastic optimisation and group-contribution methods to select chemicals with optimised properties. Each molecule is represented as a set of functional groups. The search follows an iterative procedure, where new molecules are generated, evaluated and subjected to acceptance. The evaluation stage calls upon calculation of molecular properties using-available group-contribution expressions and databases. The proposed methodology is illustrated with literature examples involving the design of refrigerants and liquid-liquid extraction solvents. The efficiency of the search and the thermodynamic models employed are validated through process simulation studies. The work reports novel molecular structures and significant improvements over conventional techniques.