In 2017 library staff were transferred from Dudley Council to Greenwhich Leisure Limited on a five-year contract and their wages fell behind council employees.
Greenwhich is described as "a charitable social enterprise for all the community" but UNISON claims it is not rewarding staff in the spirit of its own ideals.
The union revealed rates for library assistants are 12 per cent below what they would have been had the service stayed within the council, and have dropped below any other recognised council salary scale.
Branch secretary at UNISON Dudley General Branch Theresa Kelly said: “Greenwhich makes a mockery of the term ‘social enterprise’.
“Its flawed business model is based on zero-hours contracts and job cuts. The public services it pledged to maintain, such as leisure centres and libraries, are being eroded."
He added: “It’s nothing short of scandalous that an organisation which touts its progressive credentials offers wage levels so low that staff would struggle to make ends meet."
Branch chair Billy Gibson outlined the importance of libraries in Dudley: “Libraries are at the heart of communities. Many people in Dudley will be shocked that the people providing important public services are enduring poverty wages. This has to end.”
UNISON will now encourage Dudley residents, library users, and Dudley councillors to show their support for its call for Greenwhich to raise pay to at least £9.50 an hour for all employees.
A GLL spokesperson said: “Since taking on the management of Dudley Libraries, GLL has made a number of improvements to working practices including; staff rotas and structures and the removal of lone working in libraries – a practice that we inherited. We have never employed staff on Zero Hours contracts within our libraries, although we do offer flexible working, to help cover instances of staff sickness or annual leave.
“We are aware that some staff have concerns around the level of their recent pay rise and we are currently addressing these. We are committed to paying the Real Living Wage across all our facilities and are working with local staff to reach this point as soon as possible.
“ We would be delighted to meet and discuss the issue with both Theresa Kelly and Billy Gibson, as we have yet to have any direct contact from either of them.”