A sizing composition for mineral fibers containing: - a liquid phenolic resin constituted of phenol-formaldehyde and phenol-formaldehyde-methenanolamine condensates, said phenolic resin having a free formaldehyde content less than or equal to 0.1% by weight expressed with respect to the total weight of liquid, a free phenol content less than or equal to 0.5% by weight expressed with respect to the total weight of liquid, and a water dilutability, at 20°C, greater than or equal to 1000% and - 0 to 40 parts of urea per 100 parts by dry weight of the mixture constituted by the resin and the urea, - a compound capable of reacting with the free formaldehyde chosen from the group constituted of 2,4-pentanedione, 2,4-hexanedione, 3,5-heptanedione, 2,4-octanedione, acetoacetamide, acetoacetic acid, methyl acetoacetate, ethyl acetoacetate, n-propyl acetoacetate, isopropyl acetoacetate, isobutyl acetoacetate, t-butyl acetoacetate, n-hexyl acetoacetate, malonamide, malonic acid, dimethyl malonate, diethyl malonate, di-n-propyl malonate, diisopropyl malonate, di-n-butyl malonate, acetonedicarboxylic acid, dimethyl acetonedicarboxylate, 2-methyl cyanoacetate, 2-ethyl cyanoacetate, 2-n-propyl cyanoacetate, 2-isopropyl cyanoacetate, 2-n-butyl cyanoacetate, 2-isobutyl cyanoacetate, 2-tert-butyl cyanoacetate, 2-cyanoacetamide, propanedinitrile, trimethylolpropane triacetoacetate, trimethylolpropane tricyanoacetate, 1,3-cyclohexanedione and Meldrum's acid, the content of the compound capable of reacting with the formaldehyde represents 1 to 35 parts per 100 parts by dry weight of liquid resin and where appropriate of urea. - a liquid phenolic resin constituted of phenol-formaldehyde and phenol-formaldehyde-methenanolamine condensates, said phenolic resin having a free formaldehyde content less than or equal to 0.1% by weight expressed with respect to the total weight of liquid, a free phenol content less than or equal to 0.5% by weight expressed with respect to the total weight of liquid, and a water dilutability, at 20°C, greater than or equal to 1000% and - 0 to 40 parts of urea per 100 parts by dry weight of the mixture constituted by the resin and the urea, - a compound capable of reacting with the free formaldehyde chosen from the group constituted of 2,4-pentanedione, 2,4-hexanedione, 3,5-heptanedione, 2,4-octanedione, acetoacetamide, acetoacetic acid, methyl acetoacetate, ethyl acetoacetate, n-propyl acetoacetate, isopropyl acetoacetate, isobutyl acetoacetate, t-butyl acetoacetate, n-hexyl acetoacetate, malonamide, malonic acid, dimethyl malonate, diethyl malonate, di-n-propyl malonate, diisopropyl malonate, di-n-butyl malonate, acetonedicarboxylic acid, dimethyl acetonedicarboxylate, 2-methyl cyanoacetate, 2-ethyl cyanoacetate, 2-n-propyl cyanoacetate, 2-isopropyl cyanoacetate, 2-n-butyl cyanoacetate, 2-isobutyl cyanoacetate, 2-tert-butyl cyanoacetate, 2-cyanoacetamide, propanedinitrile, trimethylolpropane triacetoacetate, trimethylolpropane tricyanoacetate, 1,3-cyclohexanedione and Meldrum's acid, the content of the compound capable of reacting with the formaldehyde represents 1 to 35 parts per 100 parts by dry weight of liquid resin and where appropriate of urea.