Ferry workers are threatening the industrial action to force Holyrood ministers into protecting their pay and conditions if part of the service is broken up.
The union is concerned that the Scottish Government is putting the Ardrossan to Brodick, Wemyss Bay to Rothesay, Oban to Craignure, and Largs to Cumbrae routes out to separate tender for new contracts from October 2013.
RMT officials are due to meet Transport Minister Keith Brown on September 6 with unions pressing on with ballot arrangements.
Transport Scotland said union claims it was planning to sell off the routes were "simply untrue".
A spokesman said: "The Scottish Government is committed to supporting our remote communities."