We present evidence for the existence of a prewetting transition in the mixed region of the phase diagram of the binary liquid mixture methanol-cyclohexane. Our measurements reveal the existence of a first order thin to thick film transition as the temperature is decreased towards the separation temperature T S at a temperature T pw above T S and distinct from it. This transition is strongly hysteretic. The determined prewetting line approaches the coexistence curve tangentially. Also, the prewetting critical point is found to display 2D Ising-like criticality.