A series of precise electric deflection experiments were done on a supersonic beam of toluene molecules in several expansion conditions and field strengths. We analyzed the effect of a gradual rotational cooling on the evolution of the beam pro. le by use of the available theories of asymmetric-top dipoles in electric fields. We have recorded from non-broadened (warm) to very broad and asymmetric (cold) profiles, and discussed this effect in terms of the Stark perturbation on the rotational motion. (C) 2008 Elsevier B.V. All rights reserved.