The effect of the form of 10 types of quaternary ammonium ions (Q(+))for the phase separation phenomenon, which accompanies a charge neutralization with perfluorooctanoate ion (PFOA(-)) and Q(+), was studied, The separated phase using the spherical form of Q(+) (i.e., [4 . 4 . 4 . 4], [3 . 3 . 5 . 5], [3 . 3 . 3 . 7], and [2 . 2 . 6 . 6], where the numbers in square brackets represent the carbon numbers of four kinds of straight alkyl chains which were directly bonded to the nitrogen atom) was in the liquid state, On the other hand, the separated phase using the rod form ([2 . 2 . 2 . 10] and [1 . 1 . 1 . 13]) was in the solid state. The volume, specific gravity, and water content of the liquid-separated phase using [3 . 3 . 5 . 5] were 116 mu l, 1.27, and 6.98%, respectively, under the following experimental conditions : [Q(+)](T) = 2.30 x 10(-2) mol dm(-3), [PFOA(-)](T) = 5.75 x 10(-3) mol dm(-3), and 25 degrees C, This liquid phase was stable to physical changes such as Qi total concentration and temperature, compared with three other types of Q(+) ([4 . 4 . 4 . 4], [3 . 3 . 3 . 7], and [2 . 2 . 6 . 6]), which formed similar liquid-separated phases.