Measurements of stress and birefringence as functions of elongation are reported for two poly(dimethylsiloxane) networks differing in degrees of cross-linking. Samples swollen with dodecane, hexadecane, decalin, tetraethylmethane, and carbon tetrachloride were investigated. The dependences of the stress on elongation and on dilation are in full agreement with theoretical calculations based on concise sets of parameters which are determined within narrow ranges solely by the reduced stress [f*] in the limit αe-1 → 0, αe being the extension ratio relative to the state of rest. Data for different degrees of swelling converge in this limit to a common intercept [f*ph]> the value for the corresponding phantom network, that reflects the degree of cross-linking. Convergence to the same reduced stress at ae = 0 confirms the theoretical form of the stress-strain relationship. Through arbitrary assignment of the stress-optical coefficient [formula omitted] and reliance on parameters employed for the analysis of stress-strain data, good agreement between the birefringence An and the true stress τ is achieved. Predicted departures of An from proportionality to τ are confirmed. The observed dependence of C on the diluent in swollen samples is attributed to intermolecular correlations that effectively enhance the very small intrinsic optical anisotropy of the dimethylsiloxane unit. © 1983, American Chemical Society. All rights reserved.