Combined analysis of experimental data on monomer diffusion, phase equilibrium, and copolymerization kinetics in systems of monomers (nonyl acrylate and 2-methyl-5-vinyltetrazole) as well as their copolymers and homopolymers was carried out. The composition of the mixture in the vicinity of the growing macroradical can differ significantly from the average composition in the whole reactor volume because of consumption of the more-reactive reactant 2-methyl-5-vinyltetrazole. Nonyl acrylate exhibited limited compatibility with copolymers enriched in 2-methyl-5-vinyltetrazole and its homopolymer. Phase diagrams were obtained for the latter homopolymer. The concentration plots of the diffusion coefficients of both monomers in their copolymers of various compositions were determined. Microphase separation was observed at specific conversions in the reaction system where the composition of a copolymer and its concentration in a monomer solution approached the binodal of the phase diagram.