AbstractPenetration of the carbonized polymeric residues has been investigated and their influence on combustibility of polymers has been shown. It has been found that filtration of liquid and gas products of pyrolysis through the carbonized layer upon combustion is described by Darcy's law, the movement of liquids being due to the surface tension forces. It has been shown that the carbonized layer cannot be regarded as an important obstacle for passing of volatile and low‐viscous products into the gas phase, and that the polymeric melts can also rise through the coke due to the capillary forces. Phosphoric and some boric compounds have been found to cause a decrease in penetration of the carbonized residues, which can be one of the reasons of the flammability decrease of polymers when using such compound