The thermal decomposition rate constant (k(d)) of 2,2'-azoisobutyronitrile in acrylonitrile (AN; monomer A)-methyl methacrylate (MM; monomer B) comonomer mixtures in N,N-dimethylformamide (DMF) as a function of the comonomer mixture composition and its concentration in the solvent at 60 degrees C was studied. The dependences k(d) = f(x(A),C) [x(A) (mole fraction of A in the comonomer mixture) = A/(A + B) = A/C, where C is the comonomer mixture concentration] have a different course as a function of C: from a curve k(d) = f(x(A)) approaching the straight line (C = 2 mol.dm(-3)) to a convex curve possessing a maximum at a point x(A) = 0.7 (C = 4 mol.dm(-3)) to a curve with a flattened wide maximum within the range of x(A) = 0.2-0.8 (C = 7 mol.dm(-3)) to a curve with the shape of a lying s (C = 9 mol.dm(-3)), All the courses of the experimental dependences k(d) = f(x(A),C) can be explained with a hypothesis of initiator solvation by the comonomers AN and MM and the solvent DMF. The existing solvated forms, their relative stability constants, the thermal decomposition rate constants, and the relative contents in the system were determined.