This article presents the analysis of the processing parameters influence on the foaming behavior and cellular structure of PP-montmorillonite foams. Polypropylene nanocomposites containing 5.0 phr of an organically-modified montmorillonite (MMT) were initially melt-compounded in a twin-screw extruder with azodicarbonamide (ADC) and later foamed using a one-step compression-molding process. The cellular structure and morphology of the foams was assessed using both scanning and transmission electron microscopies. A time-dependant double-effect was observed during foaming: (1) first of all, the melt strength of the polymer, too high for shorter times, not allowing full cell growth, and too low for high foaming times due to thermal oxidation; (2) and polymer degradation, clearly observed for very high foaming times, directly affecting polymer's melt resistance. Comparatively, PP-MMT foams exhibited a broader foaming time processing window, a more isometric type of cellular structure and decreased open-cell contents, indicating an effective nucleation and cell wall stabilization induced by the exfoliated MMT particles. POLYM. ENG. SCI., 49:2400-2413, 2009. (C) 2009 Society of Plastics Engineers