Curing and destruction of crosslinked phenolformaldehyde polymers modified with a varying quantity of polycaprolactam were studied by means of IR and ESR spectra and by derivatography. In the curing process of the polymers, hexamethylenetetramine was found degrading, with formation of dimethyleneimine, and other groups causing spatial crosslinking of the polymers. Incorporation of nitrogen in the polymer chains was proved by means of IR spectra and elemental analysis. The activation energy of destruction was determined by the thermogravimetric curves, and it varied in the range of 25 to 32 kcal/mol. On the ground of the investigations made, it was presumed that in hardening and destruction of the polymers under study a definite role was played also by free-radical processes. An evidence of this was the presence of free radicals in the products during hardening and destruction as well as the kinetic data—the low activation energy of destruction.