The structure of traditional, linear phenol-resorcinol-formaldehyde (PRF) resins, urea-branched PRF resins, and phenol-resorcinol-furfural (PRFuran) resins has been investigated in depth by both matrix-assisted laser desorption/ionization time of flight (MALDI-TOF) mass spectroscopy and C-13 NMR. The structure of a variety of oligomers has been obtained, and the structures present in each of the three types of resins related to the very different percentages of resorcinol needed for their equal performance as adhesives. The oligomers type and species distribution appeared very different for each case. PRF resins performance is improved by maximizing either the proportion of resorcinol-containing oligomers or methylol-groups containing oligomers, even without any resorcinol, or both. It is equally obtained by the minimization of the relative proportion of the low reactivity Phenol (CH, Phenol) species in which resorcinol is not present, this being the most important parameter. This can be obtained by more effective use of the resorcinol by just modifying the resin manufacturing procedure. This parameter instead does not appear to be determinant in PRFuran resins. In these, it is the higher molecular weight of furfural in relation to formaldehyde that engenders for the same manufacturing procedure a correspondingly lower proportion of resorcinol in the resin. (C) 2004 Wiley Periodicals, Inc.