The characterization of oligomeric reaction products obtained by acid- or base-catalyzed condensation of phenol and formaldehyde is reported using desorption methods of ionization. Desorption electron ionization (DEI) and chemical ionization (DCI), thermospray (TSP), potassium ionization of desorbed species (K+IDS), field desorption (FD), and fast atom bombardment (FAB) techniques were performed on commercial sector or quadrupole instruments. The recorded mass spectra are interpreted in terms of the prevailing mechanism of ionization, of possible thermal damage of the samples during analysis, and of furnishing quantitative data. The methods afford meaningful spectra of oligomers synthesized by acid-catalyzed condensation (Novolak), but reliable quantitative measures of molecular weight distribution are obtainable only from FD, K+DDS, and DEI mass spectra. Only FD is applicable directly to the base-catalyzed condensation products (resol-type resin), but acetylation allows useful FD and K+IDS spectra. The molecular weight averages for the resol-type resin is, however, accurate exclusively when they are computed from the FD mass spectrum of the derivatized sample.