We have studied the addition of formaldehyde to the various phenolic monomers present in a phenol-formaldehyde system under conditions of constant temperature, pH and catalyst type. The techniques used, carbon-13 nuclear magnetic resonance and high-performance liquid chromatography, enabled us to show the effect of aromatic ring substitution on the reactivity of sites that can potentially react. We have shown from a mechanistic standpoint that the aromatic carbon in the para position is more reactive towards formaldehyde or a hydroxymethyl group than the aromatic carbon in the oi tho position. Condensation reactions involving the para hydroxymethyl group preferentially lead to the formation of compounds with a para-para methylene bridge. From a kinetic standpoint, we have demonstrated that changes in each monomer as a function of time enable the order of reactivity of these compounds towards formaldehyde to be determined under the conditions of temperature, pH and concentration employed.