Electrospinning of thermosetting elastomers is a method for developing nonwoven membranes with a microporous structure for use in dynamic applications (i.e., stretchable). These membranes have potential for chemical protective clothing applications. The most challenging concerns are the ability to control pore sizes, fiber diameter, permeability, and physical properties. We investigated the effect on fiber diameters as a ratio of two different carbon black particulate sizes. Carbon black N110 and N990 were incorporated at various percentages of total carbon black loading in a butyl rubber formulation. Higher levels of small particulates were found to give smaller and more consistent fiber diameters. Understanding of the effects of filler size and concentration on the electrospinning process will lead to the successful tailoring of nonwoven membranes with desired fiber diameter and pore size.