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SBS, which acquired 30% of Sinius in 2001, has increased its stake to 80%, gaining management control of the IT unit.
SBS provided no financial details but said it intended to expand its IT service business for financial institutions.
Sinius, with around 1,250 employees and annual sales of around €180m (£117m), specialises in providing outsourcing services to financial companies. The company, located in Düsseldorf, has 24 offices across the country.
Sinius provides Deutsche Bank, still its largest customer, with decentralised IT services, including networks and terminals, as well as central user support via helpdesks.
The Sinius deal is in line with a move at Deutsche Bank to reduce operating costs by outsourcing IT operations and focus on its core banking business.
Last month, Deutsche Bank outsourced a large chunk of its operations to IBM.