The problem of overly high contents of remnant monomers (free phenol and free formaldehyde) in phenolic foam was studied. Free phenol was characterized by iodimetry, and free formaldehyde was characterized by a hydroxylamine hydrochloride method. The effect of the phenol /formaldehyde molar ratios and the reaction temperature on the contents of the remnant monomers was examined. The effect of adding urea on the content of free formaldehyde was investigated. The effect of the foaming process on the contents of the remnant monomers was examined in detail. Phenolic foams in which the content of free phenol was 0.09% and the content of free formaldehyde was less than 0.1% were successfully prepared. A prepolymer of polyurethane, which toughened the phenolic foam, lowered the contents of free phenol and free formaldehyde. (C) 2003 Wiley Periodicals, Inc.