Novel formaldehyde novolac resins at various molar ratios of monomers (4,4'-methylenedianiline/p-nonylphenol/p-formaldehyde) were synthesized in the presence of an acid catalyst. The obtained resins were characterized by using elemental analysis, FT JR. and H-1 NMR spectroscopic methods. Subsequently, these resins may further be modified by allylation and methacrylation and may be used as cross-linking agents for epoxy resin/bismaleimide resin formulations. The curing of epoxy and bismaleimide resins with formaldehyde resins (FR) and modified FR and the thermal decomposition behavior of cross-linked resins were studied by differential scanning calorimetry (DSC) and thermogravimetric (TGA) techniques. DSC scans for the curing of epoxy resins with FR show a single exothermic peak, which implied only the occurrence of cure reactions between epoxy ring and amine protons, whereas the DSC scans for the epoxy resins, bismaleimide resins, and modified FR systems have two exothermic peaks. The cured products possess a good thermal stability and low water absorption. (C) 2011 Wiley Periodicals, Inc. Adv Polym Techn 30: 219-233, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/adv.20218