From Annex 6 to the Chicago Convention:
6.18 Aeroplanes required to be equipped with an airbourne collision aviodance system (ACAS II)
6.18.1 From 1 January 2003, all turbine-engined aeroplanes of a maximum certified take off mass in excess of 15, 000kg or authorised to carry more than 30 passengers shall be equipped with an airbourne collision aviodance system (ACAS II).
6.18.2 From 1 January 2005, all turbine-engined aeroplanes of a maximum certified take off mass in excess of 5,700kg or authorised to carry more than 19 passengers shall be equipped with an airbourne collision aviodance system (ACAS II).
6.18.3 Recommendation.- All aeroplanes should be equipped with an airbourne collision aviodance system (ACAS II)
However, ICAO procedures allowed for the earlier introduction of ACAS requirements through Regional Air Navigation Plans as has happened in Europe for aircraft over 15 000 kg.