Resorcinol and formaldehyde reactions were quantitatively monitored by means of H-1 and C-13 NMR spectroscopy at room temperature (293 K) before heat treatment leading to formation of organic gels. We found that resorcinol substitution with formaldehyde starts with an initial surprisingly rapid step followed by a more gradual depletion of the reactants. Substituted species with both monomeric and dimeric hydroxymethyl groups were observed immediately after mixing of the reagents with the proportion of formaldehyde-based solution species consumed between 30 and 50%. Substituted resorcinol species can be all accounted for by solution-phase NMR at ambient conditions before they form nano scale clusters upon heating. It can therefore be expected that the final properties of resorcinol formaldehyde gels depend not only on the composition of reaction mixtures and duration of the high temperature treatment but also on the manner and period of reagent mixing (a hitherto overlooked synthesis step), as different amounts of alternatively substituted resorcinol can be produced before heat treatment commences. (C) 2017 Elsevier Ltd. All rights reserved.