The peculiarities of ionic equilibrium in salt-containing aqueous solutions of polyampholytes (acrylic acid-2-methyl-5-vinylpyridine copolymers) of various compositions and molecular weights were studied. The protonation degree of base groups (beta(iep)), the dissociation degree of acid groups (alpha(iep)), and the ionization constant of methylvinylpyridine groups (pK(b)) for the isoelectric points of the studied polyampholytes under various ionic strength values (I) were assessed spectrophotometrically. The dependencies of alpha(iep) and pK(b) versus the copolymer composition in the absence of low molecular weight electrolyte are described by the following equations: pK(b) = 6.2-0.037z and 1g alpha(iep) = 0.27-0.0215z, where z is the molar content; of the acrylic acid units. The basicity of methylvinylpyridine groups increases in proportion to the content of acid groups at a constant ionic strength and is independent of the molecular weight and molecular weight distribution of the copolymer. The relationship between pK(b) and the ionic strength of the solution for acrylic acid-methylvinylpyridine copolymers was established: pK(b)(I) = pK(b)(t) + B . I-1/2, where pK(t)(b) is the thermodynamic ionization constant of base groups and B is 0.21 + 0.0065z. A good agreement between the experimental and theoretical (calculated from the given equation) values of the ionization constant, pK(b), of methylvinylpyridine groups for other polyampholytes (copolymers of methacrylic acid with 2-methyl-5-vinylpyridine) demonstrated that the ionic state of polyampholytes is determined by the basicity of methylvinylpyridine groups, which depends on the copolymer composition and solution ionic strength.