The substrate effect on the inhomogeneous gelation is in situ studied using various monomers: 2-acrylamide-2-methyl-1-propanesulfonic acid (AMPS), acrylic acid, and N,N ' -dimethylacrylamide on various hydrophobic substrates with different surface tensions: poly(tetrafluoroethylene), polypropylene, polyethylene, polystyrene, and poly(vinyl chloride) in aqueous solution as well as in organic solvent by means of electronic speckle pattern interferometry. When the polymerization is carried out in water, a clear interface appears in the vicinity of the hydrophobic substrates at a critical time when the auto-acceleration of the polymerization starts. The polymerization on the hydrophobic substrate is suppressed after the appearance of the interface which gives rise to a heterogeneous gel structure. The thickness of the substrate-induced inhomogeneous layer increases with the square root of the polymerization time, showing the feature of monomer diffusion. An enhanced heterogeneous polymerization occurs on the hydrophobic substrate with a lower surface tension. These effects are greatly suppressed when the polymerization is carried out in ethanol and in the presence of hydrophobic monomer, giving rise to a homogeneous gelation. The correlation between the surface tension of the substrates and the interface strength in water suggests that the substrate-induced interface formation might be associated with the high interfacial tension between the substrate and the polymerizing solution.