In recent days researchers have been trying to operate Joule-Thomson refrigerators with mixed refrigerants instead of single refrigerant as operating fluid. Usage of mixtures in the place of single refrigerant is increasing rapidly because of some favourable conditions like achieving high heat transfer coefficients, avoiding of multistage compression units, high overall efficiency of the refrigerator and a mixture of refrigerants gives more pollution free environment. Conventional vapour compression refrigeration system working with chlorofluorocarbon refrigerants will deplete the ozone layer, whereas joule Thomson refrigerators with a mixture of refrigerants will not detrimental to ozone layer hence it is more suitable for a green environment. The present paper discusses the selection of the appropriate mixture for Joule-Thomson refrigerator operating at 90 K in Gas Refrigerant supply (GRS) mode and its effect on the performance of the overall system. It is estimated that the optimization of mixture composition enhances the exergy efficiency from 31.4% to 34 % with a substantial increasein refrigeration capacity. © TJPRC Pvt. Ltd.