This article reports the results of a study on the effect of using different additives (lignin, polyethylene glycol [PEG], and polyvinyl pyrrolidone [PVP]) to fabricate ultrafiltration polysulfone (PSf) membranes. The main focus of this study was on the difference in permeation properties brought about by the absence or presence of a fabric when fabricating the membranes. Differential scanning calorimetry was used to characterize the thermal properties and was also used to predict the other membrane properties. An Instron machine was used to evaluate the mechanical properties. The bulk porosity of lignin and PVP-modified membranes was observed to be higher than that of the membranes modified with PEG. There was a strong negative correlation between the bulk porosity and the glass transition temperature irrespective of the additive used. Membranes cast on a fabric showed higher flux compared with membranes cast on glass. There was a strong positive correlation between the bulk porosity and the observed permeability regardless of whether the membrane was cast on a nonwoven fabric or on a glass plate. Pore-size distribution results showed that lignin and PVP-modified membranes had a narrow pore-size distribution ranging between 10 and 25 nm when compared with PEG-modified membranes with a pore-size distribution ranging between 2.5 and 20 nm. These results indicate that thermal, bulk porosity, and mechanical properties can be used to probe the membrane structure. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40616.