Determination by thermomechanical analysis (TMA) of the average number of degrees of freedom of polymer segments between crosslinking nodes of phenolformaldehyde (PF) resin-hardened networks indicate that additive-accelerated PF resin polycondensations and hardening presented several different acceleration mechanisms. Some additives such as sodium carbonate appear to present purely an acceleration effect on the polycondensation reaction. Other additives such as propylene carbonate appear to present an acceleration effect by also inducing an increase in the average functionality of the system, due to alternate crosslinking reactions to which the accelerator itself does participate, leading to a tighter-final network. These alternate crosslinking reactions can be of a different nature. In the propylene carbonate case, the reaction is related to a Kolbe-Schmitt reaction. In formamide, instead, it is related to its hydrolysis to formic acid and ammonia with the subsequent rapid reaction of the latter with two or more hydroxybenzyl alcohol. groups of PF resols. The rapid reaction of the - NH2 group of formamide with two hydroxybenzyl alcohol groups of PF resols, a reaction which is also characteristic of urea and methylamine, also appears likely to occur. Both liquid-and solid-phase C-13-NMR supporting evidence of the mechanisms proposed is also presented.