The acceptance period runs from 5 May to 16 June, with investors offered 0.485 UniCredit shares for each Commerzbank share, valuing the German lender at about €32 a share.
The six-week tender process will test whether Commerzbank can remain independent as UniCredit seeks to raise its holding beyond 30% from roughly 28%.
Earlier, Chief Executive Andrea Orcel said full control was not the expected outcome, after shareholders approved 470 million new shares to support the offer.
Can UniCredit's hostile bid for Commerzbank overcome staunch German political opposition?
With a low premium and fierce resistance, is UniCredit's offer a bargain or a blunder for investors?