We use a micrometer-scale optical beam deflection technique to measure the thermodiffusion coefficient D-T at room temperature (approximate to 24 degrees C) of dilute aqueous suspensions of charged polystyrene spheres with different surface functionalities. In solutions with large concentrations of monovalent salts, greater than or similar to 100 mM, the thermodiffilsion coefficients for 26 nm spheres with carboxyl functionality can be varied within the range -0.9 X 10(-7) cm(2) s(-1) K-1 < D-T < 1.5 x 10(-7) cm(2) s(-1) K-1 by changing the ionic species in solution; in this case, DT is the product of the electrophoretic mobility mu(E) and the Seebeck coefficient of the electrolyte, S-e = (Q*(C)- Q*(A))/2eT, D-T = -S-e mu E, where Q*(C) and Q*(A) are the single ion heats of transport of the cationic and. C. A. c A anionic species, respectively. In low ionic strength solutions of LiCl, less than or similar to 5 mM, and particle concentrations; less than or similar to 2 wt %, D-T is negative, independent of particle concentration and independent of the Debye length; D-T = -0.73 +/- 0.05 x 10(-7) cm(2) s(-1) K-1.