Related Rule
Ukraine
Practice Relating to Rule 108. Mercenaries
Section A. Definition of mercenaries
Ukraine’s IHL Manual (2004) states:
Mercenaries are persons who are specially recruited and take part in the hostilities in the desire for private gain. They are neither nationals of a Party to the armed conflict nor residents of territory controlled by such a Party. Mercenaries are not members of the armed forces of the belligerent Parties.
Military instructors and advisors officially sent by a State to assist in building armed forces of another State are not mercenaries provided they do not take part in hostilities personally. 
Ukraine, Manual on the Application of IHL Rules, Ministry of Defence, 11 September 2004, § 1.2.25.2.
Ukraine’s Criminal Code (2001) defines mercenary activity as “participation in armed conflicts of other States for the purpose of pecuniary compensation without authorization obtained from appropriate government authorities”. 
Ukraine, Criminal Code, 2001, Article 447(2).