Polydisperse NACl and (NH4)2SO4 salt particles were passed through a Goetz centrifuge, which transmitted only those particles smaller than the cutoff size of the centrifuge. In one set of experiments the sample from the centrifuge was passed directly into the thermal gradient cloud chamber and the threshold supersaturation for activating nuclei was recorded. In the second set of experiments the output sample from the centrifuge was passed through a mobility analyzer which transmitted a nearly monodisperse aerosol of a known size to the thermal-gradient cloud chamber. In the first experiment the threshold supersaturation agreed well with the critical supersaturation predicted by theory for particles of radii equal to the cutoff size of the Goetz centrifuge. Likewise, the critical supersaturation found for the nearly monodisperse aerosols transmitted through the mobility analyzer agreed well with that predicted by theory.