Powder X-ray diffraction and the thermoanalytical characteristics of solid solutions with compositions [(CoxNi1 – x)(H2O)4(C4H3O4)2], [(CoxZn1 – x)(H2O)4(C4H3O4)2], [(NixZn1 – x)(H2O)4(C4H3O4)2] (0 < x < 1) are compared. The decomposition of solid solutions and their constituents when heated to 500°C in a flow of He is conventionally divided into three stages. Products of the thermolysis of solid solutions are found to be Co/Ni, Co/ZnO, and Ni/Zn bimetallic nanoparticles, respectively, embedded into the polymeric matrix of composites. It is shown that the thermal decomposition of solid solutions in the Curie curve upon a second order phase transition with the formation of the two-phase region found in Co/Ni bimetallic nanoparticles is a quantum size effect. The decomposition of [(Co0.1Zn0.9)(H2O)4(С4H3O4)2] solid solution removes the Co atoms (d = 2.5 Å) embedded in channels of the ZnO structure. These atoms also serve as catalysts for the growth of a nanobrush of carbon nanotubes (CNTs) on the composite’s surface at ≤500°C, another consequence of the quantum size effect. It is concluded that using a similar catalyst of CNT growth can appreciably lower the energy of the process.