Freshfields corporate partner Jeff Roberts led the team representing UK-based TNS in the so-called merger of equals.
GfK was advised by Slaughter and May partner Simon Nicholls alongside German ally Hengeler Mueller partners Christof Jaeckle and Bernd Wirbel.
Herbert Smith is acting for Societe Generale, Royal Bank of Scotland, Barclays Capital, UniCredit and Dresdner Bank in relation to the debt facilities being used for the merger.
Partner Malcolm Hitching is leading the firm’s team with assistance from associate Tom Banks.
Once completed, the merger will create one of the world's largest market research groups with a predicted turnover of £1.9bn.
Shareholders in both companies will receive approximately 50 per cent of the new company, which will be called GfK-TNS.
The Freshfields team also included partners Stephen Hewes, Heiner Braun and Rick Van Aerssen in corporate; Colin Hargreaves and Jan Brinkmann on tax, David Winfield on finance; Nicholas Spearing on competition and Torsten Shreier on IP.
GfK-TNS will have its primary listing in London and a secondary listing on the Frankfurst Stock Exchange.