The polymeric species formed on the hydrolysis of Th(IV) from solubility experiments of $ThO_2$ in 0.1M $NaClO_4$ and $Th(OH)_4;in;0.5M;NaClO_4$ were removed by ultrafiltration using 1 nm ultrafilters. The resultant equilibrium concentrations of mono-hydroxy Th(IV) species in solutions between pH 1.5 to 13 were measured by the NAA method. From these data solubility product of thorium dioxide and the stability constants of mono-hydroxy Th(Ⅳ) were determined. The values obtained were log $K_{sp};-;50.76;{pm};0.08, log;{eta}_{1,1};12.42;{pm};0.02,;log;{eta}_{1,2};22.46;{pm};0.15, log;{eta}_{1,3};34.36;{pm};0.07,;log;{eta}_{1,4};42.58;{pm};0.08$, which are in good agreement with the values given in the literature. In addition, several dimeric species, $Th_2(OH)_2;^{6+},;Th_2(OH)_3;^{5+} and;Th_2(OH)_4;^{4+}$ have been identified.