A method of preparing phenolaldehyde foamed plastics having low water-absorbing properties comprising mixing a phenolaldehyde oligomer consisting of 100-15 parts by weight of a resol resin and 0-85 parts by weight of a novolac resin, with an acidic catalyst selected consisting essentially of (A) an oligomeric product obtained by the treatment of an aromatic sulphoacid with formaldehyde, or (B) an oligomeric product obtained by the treatment of an aromatic sulphoacid with formaldehyde and a nitrogen-containing compound selected from the group consisting of an amine and an amide, a complex-forming metal selected from the group consisting of cobalt, magnesium, manganese, vanadium, chromium, nickel, copper, iron, titanium, and lead and in the form of its oxide, hydroxide, salt, or mixtures thereof, a surfactant and a foaming agent, and then foaming and curing the mixture at a temperature up to 100.degree. C.