Using the half leaky guided mode geometry, we have studied the electroclinic effect using a thin homogeneously aligned layer of C8, a material having a first order SAto S*Ctransition in the bulk. Application of voltage in the SAphase produces a small (< 12°), but easily measured director twist which is characterized over a range of temperatures in the SAphase. Mean field theory is used to interpret the data recorded. At low applied fields (⩽6 V μm−1), a linear voltage dependence of induced twist is found. For higher applied fields there is a non-linear dependence which is caused by an induced first order SAto S*Ctransition at these fields. Fitting the voltage dependent induced twist yields the first three Landau coefficients for this transition. The voltage dependent results also provide a procedure for determining the SAto S*Cphase transition.