CINMan has about £16bn in assets under management and is estimated to be worth £300m to £400m.
British Coal said that some 60 organisations, including foreign groups, had expressed interest in CINMan. Philip Hutchinson, director for legal affairs, said the sale was not driven by proceeds and that those bidding would need to meet a "whole series of requirements".
He said: "There are half a million pensioners and deferred pensioners. It is their interests and the continued good performance of the funds which we need to secure. It is the quality of service we are interested in - a safe pair of hands for the £16bn under management."
He added that as "an essential requirement" the new owners would have to show commitment to the existing management. Some CINVen staff are already thought to have told British Coal that they will defect if their independence is compromised in any way and may seek finance to set up a rival venture capital organisation.
City analysts say that likely bidders for CINMan include Mercury Asset Management, Morgan Grenfell, Barclays de Zoete Wedd and Schroders.
CINMan manages most of the investments of the British Coal Staff Superannuation Scheme and the Mineworkers Pensions Scheme. The schemes, which are being closed to contributing members following the sale of the bulk of British Coal assets, provide that pensions and deferred pensions will be increased each year in line with the retail price index.