Ion engines accelerate electrically charged plasma through two optic grids and emit the ions as exhaust. The process facilitates propulsion without use of chemical propellants. Braided carbon fiber reinforced composite (C-(f)/C) optics are presently being considered for use as the accelerator and screen grids in ion propulsion engines. In this study the mechanical behavior of four candidate tow configurations proposed for the grids of NASA's Evolutionary Xenon Thruster (NEXT) were examined. A new bi-axial optical extensometer based on Digital Image Correlation (DIC) was developed and employed in determining the in-plane strain distribution resulting from uniaxial tension. The effective elastic modulus ranged from 4 GPa to 10 GPa at the onset of deformation. The stiffness either increased or decreased with further elongation as a result of bending of the axial tows and corresponding unit cell distortion. The transverse strain and Poisson's ratio of the panels were found to be a function of the tow dimensions and bonding between longitudinal and transverse tows. (C) 2004 Kluwer Academic Publishers.