Sequential amide formation between poly(vinylamine-co-N-vinylformamide)s [poly(VAm-co-NVF)s] with VAm contents of 20, 28, and 42 mol % plus polyAAc using a water-soluble carbodiimide was studied to prepare ultrathin hydrogels on a solid substrate. The analysis of assembly amounts at each assembly step using a quartz crystal microbalance revealed that at least 28 mol % of VAm was essential for stepwise reaction of both polymers on the substrate. The assembly amount was regulated not only by the VAm content but also by the concentration of the copolymer in the reaction solution. Reflection- absorption spectra of the assemblies showed typical peaks assigned to both polymers. The thickness of the assemblies, which was analyzed in the scratching mode using atomic force microscopy, increased in water to form ultrathin hydrogels. The increase in thickness was dependent on the VAm content, and the assembly prepared from poly(VAm-co-NVF) with a 42 mol % VAm content swelled with more water molecules compared to that with a 28 mol % VAm. Analysis using cyclic voltammetry revealed that Fe(CN)(6)(3-) readily permeated the ultrathin hydrogels by the process of diffusion. The permeability was also dependent on the VAm content. Collectively, these results indicated that ion-permeable ultrathin hydrogels were successfully prepared on a solid substrate.