The permeate flux and retention of aqueous solutions of poly(ethylene glycols) (PEG) with different molecular weights ranging from 4000 to 35,000 Da have been investigated using various compositions such as 100/0, 90/10, 80/20, and 70/30 wt% of cellulose acetate (CA)/sulfonated poly (etheretherketone) (SPEEK) ultrafiltration blend membranes. The factors affecting the rejection rate and permeate flux such as molecular weight of PEGs, concentration of the solute, composition of the membranes, and transmembrane pressures have been studied. It is seen that the increase in the concentration of PEG results in the decreased permeate flux and increased rejection for increasing CA content in the membranes. A similar observation in the flux and rejection was made for increasing the molecular weight of PEGS. Further, the mass transfer, diffusion, and true retention coefficients of the solute have been studied with different operating variables like molecular weight and concentration of PEGS. An increase in the molecular weight of PEGS results in the decrease of mass transfer and diffusion coefficients and increase of the true retention coefficient. A reverse trend is observed with increasing concentrations of PEG.