Heterocoagulation of large and small oppositely charged colloid particles, accompanied by spreading of small beads over the surface of large spheres, offers a promising alternative to synthesis of core-shell particles via interfacial polymerization. In this paper, conditions required for complete spreading of the shell-forming polymer over the surface of the core-forming material (CFM) are predicted in terms of a critical distance, x(cr), between the small particles on the surface of the CFM. The theoretical value of x(cr) is tested in experiments conducted for polypyrrole/polyacrylic and silica-titanyl sulfate/polyacrylic heterocoagulate units.