The invention relates to a surface treatment composition for paper, board or other fibrous webs. The composition of the invention comprises particles which comprise an active material and a supporting material. The active material comprises a salt or a multivalent metal, such as a divalent or trivalent metal. In accordance with the invention, the supporting material is adapted to release the active material from the particles when subjected to heat and/or pressure and/or a change in pH. Consequently, the active material's adverse effects on the rheology of the composition are avoided while its desired effects on the surface characteristics are retained or enhanced.