Abstract The curing process of commercial phenolic, lignin–phenolic and modified lignin–phenolic–novolac resins with hexamethylenetetramine (HMTA) 9 wt% was studied by differential scanning calorimetry (DSC) and thermal mechanical analysis (TMA). Firstly, a demonstration is provided to show that the resin samples studied are associated with an isoconversional principle. Secondly, the isoconversional methods Kissinger–Akahira–Sunose and Friedman are applied to study resin cure using HMTA. These methods yield a dependence of the activation energy on the extent of the cure for three sample resins. Note that either method shows different results in activation energy at low and high conversional degrees. But, both methods have a little variation in activation energy for a curing degree ranging from 10 to 90%.