Hourly paid staff at the University of Aberdeen have launched a collective grievance over their pay.
Eight teaching assistants from the School of Social Sciences claim that the school has failed to pay them for the hours they have actually worked.
The eight are seeking additional pay, backdated to the start of this term, as well as a commitment to implement the university's policy on hourly paid staff.
Under the policy, which was agreed with the University and College Union in 2006, staff are paid a minimum hourly rate of £11.22 and the rates equate to points on the national pay spine. Individual schools determine how many hours are offered to teaching assistants.
But Alex Arthur, president of Aberdeen UCU, said that some managers had been using student contact hours as a basis to calculate staff pay.
"For some staff this is a gross underestimate of the hours actually worked," he said. Teaching assistants' jobs are not properly defined in all cases and they have expanded beyond the paid-for hours, he suggested.
Assistants are being asked to mark assessments, provide pastoral support and, in some cases, keep office hours for student queries.
Penny Howard, branch UCU representative for postgraduates, said that the eight complainants were providing "volunteer labour". Tutorial groups that contained ten students in 2004 now had up to 18, despite a "theoretical limit" of 15 students, she added.
An Aberdeen spokesman said that its grievance committee was due to hear the matter this month, so the university could not comment on the case in detail. But he added: "Our procedures permit hourly paid staff to be paid for those hours reasonably required to undertake the duties of the role.
"The assessment of the number of hours to be offered to individuals is determined by heads of school and agreed with the individual. Individuals are paid for activities such as preparation time, exam marking and administration in addition to the required delivery of material."