The present invention relates to a method of reducing the content of free formaldehyde in phenol-formaldehyde resins, wherein this method comprises the step of treating the phenol-formaldehyde resin starting material with at least one oxidizing agent. The present invention further relates to the phenol-formaldehyde resin resulting from this process and its use as a binder in the manufacture of mineral wool insulation products of low formaldehyde emissions. The resulting phenol-formaldehyde resin has a free formaldehyde content of lower than 0.5 weight % determined according to ISO 11402; the pH is in the range of 7 to 10, the dilutability is more than 20 times in demineralized water at 20°C, the viscosity is below 100 mPa.s at 20°C, and the phenol-formaldehyde condensates, contained in the resin, consist of phenol and formaldehyde.