For most chemical reactions, the rate coefficient, k, is independent of the extent of reaction, alpha, and varies with the temperature, T, according to the Arrhenius equation, but, for melt polymerization at a fixed T, the diffusion coefficient, D, of the reactant pairs decreases as alpha increases, and for viscous liquids in general D varies with T according to the Vogel-Fulcher-Tammann equation. We propose that a change in the mass-controlled to diffusion-controlled reaction kinetics during a melt's polymerization would be seen first as k begins to decrease with increase in alpha, and second as the temperature dependence of k for a fixed alpha deviates from the Arrhenius to the Vogel-Fulcher-Tammann type. The range of alpha and T over which this change occurs may be determined by calorimetry or related experiments. Polymerization of a liquid mixture to a random network structure has been studied by calorimetry. It is shown that, (i) the ln(k) against alpha plot at a fixed T bends downwards progressively more as alpha increases, and (ii) over a given range of T, the ln(k) against 1/T plot at a fixed alpha is a straight line when alpha is low, and bends downwards when alpha is high. The onset temperature of this bend increases as alpha is increased. Thus the gradual onset of diffusion control varies with both alpha and T. The simulated dalpha/dt for mass-controlled kinetics is higher than that for diffusion-controlled kinetics up to a certain time and lower thereafter. The ratio of dalpha/dt for the two kinetics shows a local maximum at a certain time. The procedure developed here would be useful for studying diffusion control in biological processes.