The present invention relates to a catalyst masterbatch for cross-linking a polyolefin having cross-linkable silicon-containing groups. The catalyst masterbatch includes both of a Brønsted acid and/or Brønsted acid anhydride (A); and a polyolefin (B) containing Brønsted acid and/or Brønsted acid anhydride groups. The invention also encompasses compositions of a polyolefin having cross-linkable silicon-containing groups that include the catalyst masterbatch, use of the catalyst masterbatch in a cross-linking reaction of silicon-containing polyolefins, and a cable layer that includes a polymer made using the catalyst masterbatch.