In order to investigate colloidal behavior of the oxides and/or hydroxides of Ce, La, Nd, and Th, which encounter frequently in the processing of monazite, their zeta-potentials (ξ-potentials) were primarily determined by a zeta-meter as a function of pH. Also, critical ξ-potentials of those colloids were categorized through settling tests aided by light transmittant measurements. The zero point of charges (z.p.c,) for ThO₂, CeO₂, La(OH)₃, and Nd(OH)₃ lay respectively at pH 6.7, 2.1 & 11.5, 8.8, and 10.5. The critical ξ-potentials obtained were ±15mV(ThO₂), ±8mV(CeO₂), ±6mV(La(OH)₃), and ±10mV(Nd(OH)₃). From the acquired informations, an attempt to separate physically the mixture of oxides and/or hydroxides by selective flocculation was performed, which gave successful results.