Block copolymer (BCP) ultrafiltration (UF) membranes derived from a hybrid process of BCP self assembly and non-solvent induced phase separation (SNIPS) exhibit an asymmetric structure consisting of ordered pores in a selective skin layer above a highly permeable substructure. In this work, we investigate relative humidity (RH) as a casting condition that influences poly(isoprene-b-styrene-b-4-vinylpyridine) (ISV) membrane surface structure in terms of order and uniformity. An optimum RH of 40-45% is determined to produce membranes characterized by a high density of square packed pores with a narrow pore size distribution. Membranes cast at lower and higher RH show a drop off in order and uniformity. The RH dependent structural changes correlate well with small molar mass dye solute (methyl orange; molar mass of 327 g/mol) diffusion rates. Membranes cast at 40% RH reveal higher diffusivity than when cast at 75% RH. The rate of permeation is further controlled via pH dependent pore closure as well as through ISV terpolymer molecular architecture with increasing solute diffusivity observed in membranes cast from increasing terpolymer molar mass. Experimental findings on small molar mass solute permeability can only be accounted for when compared with theoretical predictions from a hydrodynamic theory combined with the effects of tortuosity when a simplified membrane structure is assumed. Results suggest that SNIPS derived BCP membranes may have potential for applications in drug delivery. (C) 2017 Elsevier Ltd. All rights reserved.