Choice, which broadcasts on two frequencies, 96.9FM and 107.1FM, has a potential reach of 5.8m listeners. In the last Rajar for the three months to June 30, Choice's combined audience was 277,000 with a London market share of 0.8%.
The move comes as media companies are piling increasing pressure on the UK government to remove regulatory barriers preventing radio and TV companies expanding.
Capital Radio chief executive David Mansfield said, "Choice is an excellent investment for Capital. It brings to us a stake in a brand that has exciting growth potential and operates in an important music genre. We anticipate increasing our shareholding and becoming more involved with the business when the Radio Authority proposals for relaxing ownership gain government approval."
Choice owns four digital licences in the UK -- London, the North West, the North East and the Severn Estuary -- and a 7% share in the MXR digital radio consortium. The deal values Tainside at £16m.
Capital Radio Group already owns and operates Capital Radio and Xfm in London.
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