The present invention relates to a method for cross-linking a polymer article which comprises a cross-linkable polymer with hydrolyzable silane groups and a crystallinity level of at least 30%. The cross-linking is performed in the presence of a silanol condensation catalyst comprising a compound of formula (I): ArSO₃H or a precursor thereof, Ar being a hydrocarbyl substituted aromatic group comprising at least 14 carbon atoms.