A series of in situ polymerized CNT/PI nanocomposite materials synthesized at a fixed diamine-to-dianhydride monomer feed ratio with varying concentrations of added –COOH- functionalized CNT is studied. It is found that the glass transition temperature of the nanocomposite varies non-monotonically with CNT concentration. To determine the origin of this behavior, the molecular weights of the matrix polymers are measured by the intrinsic-viscosity method. The functional-group density of the CNT material is determined by potentiometric titration. The results confirm that the glass transition of the CNT/PI system depends on the concentration of the CNT filler through its influence on the functional group stoichiometry and thus on the molecular weight of the polycondensation product.