The deal covers the digital music stations The Hits, which has a Freeview slot, The Box, Smash Hits, Kerrang!, Q, Kiss and Magic. Emap said that the joint venture would look at how to exploit digital growth, including video-on-demand, and that the move capitalised on Emap's television brands and Channel 4's music content and expertise in digital channel and platform management.
There would also be cross-promotional opportunities for both companies, Emap said.
It is believed the plan is to close The Hits and launch Channel 4 brand 4Music on Freeview and other digital platforms in January, although this was not confirmed today.
Sky has a four-year contract with Emap that gives it rights to sell airtime on the Emap music television station, and its revenues could be hit by the deal. It has been reported to have called in lawyers to see if it can take any action should The Hits be closed. This morning, Emap would only say that it was "business as usual" with Sky, and could not comment further on the future of the deal.
Paul Keenan, chief executive of Emap Consumer Media, said: "This alliance between Emap and Channel 4 will enable us to increase our music brands' presence on new and emerging platforms, including video-on-demand, opening up new opportunities outside of the traditional broadcast market for both audience and revenue growth.
"Emap and Channel 4 are like-minded partners and together we make a formidable force in an otherwise fragmenting and challenging market."
Emap bought The Box in 1996 for £6m and began a series of launches, some of which were based on its music magazine brands, such as the now-defunct Smash Hits as well as Kerrang!.
In the year to March 31 2007, Emap's music TV business had revenue of £27m and operating profit of £7m.
The deal ties in with Emap's moves to focus more on business-to-business publishing, as well as Channel 4's development of its music programming.
Andy Duncan, Channel 4's chief executive, said: "Music is an important part of Channel 4's strategy for future growth and we intend to further strengthen our cross-platform presence with the launch of three 4-branded radio stations from next year."