The effects of feed S/Cd ratio and feed thiophenol (PhSH) concentration on a PhSH modification reaction and the redispersion property of CdS ultrafine particles prepared in situ in reverse micellar systems are investigated on the basis of the stoichiometric balance of the reactants, The concentration of PhSH connected with the surface of CdS ultrafine particles (C-SHcon) is estimated as a function of the feed S/Cd ratio, y. The experimental relationship between particle redispersion ratio R and y can be explained by assuming that R is a function of C-SHcon. The effect of decrease in the feed PhSH concentration on R is investigated using the calculated values of C-SHcon. The calculated R profile agrees very well with the previous experimental data taken without adding pyridine, Pyridine addition is likely to be effective for increasing the value of R by preventing the irreversible coagulation of CdS particles.