A study is presented to elucidate how the trivalent actinide ions become incorporated into the formation of aluminosilicate colloids. The acidic Al solution of varying concentration from 10(-5) to 10(-3) Mol l(-1), containing the AM(3+) or Cm3+ ion at 4.9 x 10(-8) Mol l(-1), is titrated with the alkaline Si solution of different concentrations from 10(-5) to 10(-3) mol l(-1) to arrive at each prefixed pH from 4 to 9. The aluminosilicate precipitate is separated by filtration at 450 nm pore size. The filtrate containing ionic and colloidal species is subsequently put into filtration at 10-kDa-pore size (ca. 1.5 nm) to separate aluminosilicate colloids from ionic species in solution. The distribution of Am or Cm in the three different phases: precipitate, colloids and ionic species is analysed by radiometric measurement and the optimal conditions are ascertained for the formation of colloid-borne Am or Cm species. The particle size of the aluminosilicate colloids observed by atomic force microscope (AFM) appears to be 5-10 nm height and 10-50 nm length for most particles, whereas the average hard sphere diameter of preponderant particles evaluated by laser-induced breakdown detection (LIBD) appears to be 10-50 nm with an approximate mass concentration of 10-50 ppb. According to these data, the colloid number density may range from 10(11) to 10(14) particles per litre solution. XPS and EDX analyses on colloids result in an atomic ratio of Al/Si to an average value of 0.7, suggesting that it may vary from 0.5 to 1.0. The speciation of colloids by time-resolved laser fluorescence spectroscopy (TRLFS) shows the formation of two different colloid-borne Cm species as coordinated with bidentate and tridentate bindings within the aluminosilicate structure. The latter species becomes predominant at pH greater than or equal to 6. A desorption experiment reveals that Am incorporated into aluminosilicate colloids is not dissociable at pH 7 and 9, indicating the formation of a stable colloid-borne Am species. (C) 2002 Elsevier Science B.V. All rights reserved.