The efficacy of the surface treatment of particulate fillers depends on the chemical character of the components, on the method and conditions of the treatment, and on the amount of the treating agent. Here, the ultra-fine calcium carbonate is surface treated with 1, 2. 3 and 4 wt.% polyacrylic acid (PAA) synthesized by ourselves, which has strong ionic interaction and is an efficient surface modifier. The PAA coated filler is submitted to the measurement of the surface bonded amount, bonding efficacy, X-ray photoelectron spectroscopy (XPS) and inverse gas chromatography. Maximum efficacy is expected at the monolayer coverage of the surface. which is about 0.6 wt.% according to the calculation based on the way they are aligned and is basically in agreement with the "substrate overlayer" model based on the mole ratio of C-286 and C-290 taking no account of the possible underestimation because of the inaccuracy or because of the CH, contamination present originally on the CaCO3. The initial decrease of the mole ratio of C-290/O and C-290/Ca with the surface bonded PAA may indicate that the bonding interaction between the polymer and the filler surface is the leaving of one molecular carbon dioxide. The IGC measurement shows that there is a considerable surface tension falling in the case of the PAA modified filler compared with the reference. An abnormal high surface energy in the case of filler treated with 4% PAA is observed. (C) 2003 Elsevier B.V. All rights reserved.