A polypyrrole (PPy)/Na+-montmorillonite (MMT) clay composite was prepared by an electropolymerization method on a gold substrate. The results indicate that the nucleation and growth are instantaneous two-dimensional and three-dimensional processes before and after nuclei overlapping, respectively, for the composite electropolymerization, which are distinguishable from the nucleation and growth mechanism of pure PPy electropolymerization before nuclei overlapping. The surface morphology of the prepared composite is denser and more compact. The thermal stability of PPy is improved, as shown from thermogravimetric analysis, due to the modification of the incorporated clay. Also, the conductivity of the composite was significantly increased (similar to20 times) and it can depress aging, in comparison to pure PPy. (C) 2002 Elsevier Science B.V. All rights reserved.