The thermal decomposition rate constant of AIBN (k(d)) in N,N-dimethylformamide (DMF)/methyl methacrylate (MM) mixtures of various compositions at 60 degrees C is studied. The hd value is 6.45 X 10(-4) min-l for pure DMF and 7.20 X 10(-4) min(-1) for pure methyl methacrylate. The hd values of DMF/MM mixtures were found to be dependent on the mixture composition. This dependence is not a linear function of the monomer mole fraction, but has a minimum at ca. 20-30 mol % of MM, The relationship between the AIBN decomposition rate constant and the monomer mole fraction was interpreted on the basis of solvation of the initiator molecules.