The broadcasting union Bectu has written a letter of complaint and says it will contact HM Revenue and Customs, which is responsible for enforcing the national minimum wage.
General secretary Gerry Morrissey said the organisers of the Music Of Black Origin awards were "steeped" in equality and should not be "exploiting" people.
The union called for anyone who has been taken on to work on the awards, which are on October 19 at The Hydro, to be paid at least the minimum wage.
Mr Morrissey said in a letter: "For an organisation rooted in the principles of equality we are very disappointed that Mobo is again seeking to exploit weak enforcement of the national minimum wage by choosing not to pay its interns.
"This situation is even more incredible given the highly commercial nature of the event and its connections to music multi-nationals."
The union said it had contacted the organisers in 2011 over the use of unpaid volunteers.
The Mobos have been working with Young Scot volunteers.
A Mobo spokesman said: "To help give unskilled workers the opportunity to work within an entertainment environment and to try and support these requests, we invited a number of volunteers into the office, enabling them to be guided by professionals and gain invaluable skills, whilst covering their expenses.
"Now we are in the production stage of the event, we do not have any volunteers helping in the office."
Performers at next month's awards will include So Solid Crew, Tinie Tempah, Rudimental, Laura Mvula and Iggy Azalea.
The Mobos were held in Glasgow in 2009 and 2011 and will also be staged in 2015.