Stepwise assembly of isotactic poly(methyl methacrylate) and syndiotactic poly(methacrylic acid) fromacetonitrile and mixed acetonitrile/water solutions, respectively, onto a gold substrate was studied.Quantitative quartz crystal microbalance analysis of the molar ratio (monomer unit) between assembledpolymers suggested stepwise stereocomplex formation. The assembled amount and the assembly ratiobetween PMAA and PMAA were dependent on the acetonitrile content of the poly(methacrylic acid) solution.Infrared spectra of the assembly and cast films of each polymer also showed stereocomplex formation inthe assembly. Atomic force microscopic observation showed the assembled polymer to have a molecularlysmooth surface. The solvent species used was an important factor in stereocomplex formation at theliquid−polymer film interface.