The preparation and characterization of high-molecular weight diphenyl oxide-and diphenyl sulfide-novolacs are described. The polymerization of diphenyl oxide with 2 equivalence of formaldehyde proceeded to give the corresponding polymer (2) (M-n: 160,000, M-w/M-n: 29.1) in good yield. Further, an ultra high-molecular weight fraction (M-w > 1,000,000) was observed in the GPC trace of 2. The developed polymers can be considered as a new class of aromatic polymers that exhibit both novolac-like and engineering plastic properties and are thus promising compounds for material-engineering applications. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 118: 1651-1657, 2010