Abstract: A method of coating glass fibres to protect them from deterioration in an alkaline environment such as a Portland cement matrix, in which the glass fibres are coated with a composition containing a water-soluble film-forming material having free aliphatic hydroxyl groups in the molecule, a water-soluble ester formed by reaction of a tri-hydroxy- or di-hydroxy-substituted aromatic carboxylic acid with an alcohol having at least two hydroxyl groups in the molecule, and a cross-linking agent, the composition is cured and dried at elevated temperature so as to cross-link hydroxyl groups of the film-forming material and thereby form a thermoset film coating on the glass fibres which also retains the ester. The ester acts as a protective material to protect the glass fibres from alkaline attack.