Schoonmaker, an American who joined Emap in 1983, has headed what is now Emap's radio and digital television business, Emap Performance, for more than 10 years.
No reason has been given for Schoonmaker's decision to leave and as yet there are no details available about his next move.
On leaving Emap, Schoonmaker will continue his long association with the firm, working as a consultant to Emap in the development of its digital radio and digital music TV interests.
Tom Moloney, chief executive of Emap, said: "Tim's decision to move on is clearly a significant moment for the group, but I am delighted we have been able to retain the benefit of Tim's knowledge and experience. His advice will be invaluable at what is a pivotal time for Emap in digital radio and music TV."
In an interview with ±±¾©Èü³µpk10 in June 2000, Schoonmaker maintained that he had no real desire to leave Emap. "I've reinvented myself three or four times. There's a relaxed ambition at Emap which I like."
Schoonmaker initially joined Emap's Micronet venture and when that was sold to BT, Schoonmaker became the head of strategic planning looking at acquisitions for Emap, before running one of its regional newspaper groups in Sussex.
He was behind Emap's win of the Kiss FM licence in 1991, which marked the start of Emap's expansion into radio and later digital television.
David Arculus, the former chairman of IPC Magazines who gave Schoonmaker his first job at Emap, said: "He's quite cerebral. He spends quite a lot of time thinking through issues, but my experience of Tim is that he normally comes up with the right solution."
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