The use of self-propagating frontal polymerization to produce polymer blends has been investigated. A homogeneous solution of monomer, polymer (of a second monomer), and initiator is initially heated at a specific location to start a polymerization front that propagates along the system. The obtained polymerization reaction is very fast compared with diffusion, sea that the two types of macromolecules remain homogeneously distributed, as in the original monomer/polymer solution. In the case of two incompatible polymers, the system has no time to reach the equilibrium condition where phase separation occurs and a polymer blend is obtained in the form of a metastable solution. Applications to two incompatible pairs (methyl methacrylate-styrene and methyl methacrylate-methacrylic acid) are discussed. Moreover, it has been found that the presence of inert polymer in the initial mixture improves the process performance, leading to more stable propagation fronts, lower front temperatures, and higher monomer conversions.