A soil fulvic acid isolated from a northern coniferous forest (NCFA) was fractionated by ultraflltration into tln'ce different molecular sizes ranging from less than 500 to 10000 daltons. The ¢onfmmatiom changes of this soil fulvic acid, NCFA, and its molecular weight (MW) fractions were studied using flumescence polarization as a function of pH and concentration of added ionic inorganic aluminum, Polarization values versus aluminum concentration increased for the lowest molecular weight fraction, whereas they decreased for the higher molecular weight fractions. The change in the polarization values suggests that the lower molecular weight fraction of the fulvic acid molecule does not change conformation when complcxed to aluminum, whereas the higher molecular weight fractions changes conformation when complexed to ~minum. fluorescence intensity versus aluminum concentration increased for all the ~ns indicating that the rigidity of the fulvic acid molecule increases as aluminum binds to it. Stability constants were calculated for the aluminum-fuivate o3~plexe~ using anisotropy values calculated from the polarization data. The log of the stability constants for the ahnninum-fulv'~e complex varied between 4.9 to 5.2 over a pH range of 3.0 to 4.0. The log of the slability constants for the ainminum-fulva~ MW fractions varied between 4.2 to 5.1 at a constant pH of 3.5.