The mechanical stability of inactive polymeric components (e.g. separator and binder) can play an important role in the long term performance of lithium-ion batteries. Here we investigate the effects of electrolyte solvents on the mechanical properties of a polypropylene battery separator through experimental measurements of thickness and elastic modulus of separator samples immersed in different solvent environments. We find that certain electrolyte solvents such as dimethyl carbonate, diethyl carbonate, and ethyl acetate cause noticeable softening of the separator. However, in other solvent environments such as propylene carbonate and acetonitrile, the separator retains the mechanical properties of a dry material. We show that the mechanical property reduction can be attributed to polymer swelling and explain these observations in the context of the Hildebrand solubility and Flory-Huggins interaction parameters. The solubility/interaction parameter analysis provides a straightforward method for predicting the in situ mechanical behavior of polymer separators in solvent environments. The relationships discussed herein can be used to screen and identify mechanically-stable polymer and electrolyte solvent pairs for use in lithium-ion batteries designed for long life.