The thermal decomposition rate constant of AIBN (k(d)) in N,N-dimethylformamide (DMF)/acrylonitrile (AN) mixtures of various compositions at 60 degrees C is studied. The k(d) value is (6.45 +/- 0.3) X 10(-4) min(-1) for pure DMF and (7.20 +/- 0.3) X 10(-4) min(-1) for pure acrylonitrile. The k(d) values of DMF/AN mixtures were found to be dependent on the mixture composition. This dependence is not a linear function of the monomer mole fraction (x(M)), but has a minimum at ca. 70 mol % of AN. The relationship k(d) = f(x(M)) has been interpreted on the basis of the hypothesis of initiator solvation by monomer AN and solvent DMF.