In this work, a molecular solvent design procedure for separation processes such as liquid-liquid extraction, absorption, etc. is presented. Solvent molecular structures built from chosen sets of groups are generated in the three dimensional space by the use of a molecular graphics system (MGS) according to user specified combinatorial rules. The screening of the generated solvent candidates is performed according to distribution coefficients, selectivities, capacities, solvent losses etc, and which are the primary and most important criteria for, particularly, liquid-liquid extraction. The design procedure is based upon the UNIFAC model estimates of the activity coefficients at infinite dilution of the different compounds present in the extract and raffinate phases, where calculated molecular, rather than the existing liquid-liquid group interaction parameters, are used. The method is tested with two different industrial examples namely toluene/n-heptane and phenol/water systems.