Electric-field-dependent dynamic light scattering (EFDLS) was used to determine the twist elastic constant and twist viscosity of miscible nematic solutions of side-chain liquid crystal polysiloxanes (SCLCP) in 4＇-(pentyloxy)-4-cyanobiphenyl (5OCB). The results show that addition of SCLCP leads to a decrease in the twist relaxation rate, which derives mainly from an increase in twist viscosity. The magnitude of the twist viscosity increment varies with molecular weight and spacer length, proportional to changes in the molecular hydrodynamic volume of the polymer. These results are consistent with the Brochard model of the viscous loss introduced by the dissolved polymer, modified by inclusion of an additional dissipation mechanism, postulated to arise from an elastic torque between director rotation and LCP orientation.