Phenol-formaldehyde resins, which are principally intended for the production of resin foams for use as insulating materials, in particular fire-resistant foams, are produced by reacting a sulphonated novolac resin, which has been condensed from at least one phenol and formaldehyde and subsequently sulphonated, with a resole resin. The novolac resin may be condensed from 1 mole of the phenol or phenols and between 0.3 and 1.0 moles formaldehyde in the presence of an organic acid, such as oxalic acid, and subsequently sulphonated with 0.1 to 1.5 moles 98% sulphuric acid at a temperature not exceeding 150*C. The resole resin may be condensed from 1 mole of a phenol or phenols and 1 to 3 moles formaldehyde in the presence of an alkaline catalyst. An aqueous dispersion of the resole resin is reacted with the sulphonated novolac resin, the viscosities of the two reactants being adjusted to be of the same order of magnitude, e.g. by diluting the sulphonation product with water. A surfactant and a foaming agent are added to the reaction mixture to cause foam formation.