Wearn is taking time out to travel and will return to BARB on a consultancy basis in the autumn.
Bolus, who takes up his new role in April, has been research manager at BARB since March 2003.
He joined BARB from Zenith Media where he was head of research. Before that, he was production audience research manager at Carlton Television.
BARB’s chief executive Bjarne Thelin said: "I am delighted to appoint Simon as BARB's new research director. He has first-class research skills, experience of many aspects of the media industry from support of media planning and buying to analysing and interpreting schedule performance, and an extensive knowledge of the BARB system."
BARB's new television audience measurement service went live this month. It aims to give users new methodologies, improved geographical data and a new viewing panel.