Abstract: A substrate for an alkaline battery separator is made of a light weight, porous, heat bonded, synthetic organic sheet material having a basis weight of less than about 35 gsm and a thickness of less than about 200 microns. The major fibrous component is synthetic pulp comprising thermoplastic polyolefin fibers having a prefused microfibrillar structure similar to wood pulp. The minor fibrous component is a high tenacity polyamide fiber having a fiber length greater than about 6 mm. The heat bonding by partial fusion of the microfibrillar polyolefin is sufficient to impart to the sheet material a wet tensile strength of at least 400 g/in. width while permitting retention of air permeability of about 100 liters per minute and more. The substrate exhibits excellent receptivity to impregnation by pore-forming battery separator coatings and is particularly well suited for use in nickel-zinc batteries.