Abstract: A battery separator comprising a microporous sheet formed substantially of a mixture of polymeric constituents. One of the polymeric constituents is an ultra high molecular weight polyolefin having a standard load melt index of substantially 0 and an intrinsic viscosity greater than about 3.0. The other polymeric constituent is either a copolymer of an olefin and an ethylenically unsaturated monocarboxylic acid selected from the group consisting of acrylic acid and methacrylic acid, or a blend of a low molecular weight polyolefin and a polymer of acrylic or methacrylic acid.