Acid whey from Greek-style yogurt (GAW) has become abundantly available in recent years. Due to its low solids content, low pH, high lactose and mineral content, GAW is challenging to transport and process. In this study, a combination of reverse osmosis (RO) and forward osmosis (FO) was developed for the nonthermal concentration of GAW. Permeate flux, retentate concentration, and specific energy were evaluated for the individual and combination processes. GAW (6.6 degrees Brix) was concentrated to 19.6 degrees Brix by RO (3 x), then to 40.2 degrees Brix (6 x) by FO. During the concentration, permeate fluxes dropped from 33.2 L/m(2)h to 2.6 L/m(2)h for RO and from 3.6 L/ m(2)h to 1.6 L/m(2)h for FO. The specific energy consumption was 0.29 kWh/kg water for RO and 0.65 kWh/kg water for FO, from which half pertains to the osmotic agent regeneration. This combination process can become an efficient nonthermal method for concentrating challenging or sensitive liquid foods and beverages.