The magnetic properties and the magnetic phase diagram of the pseudobinary Co1-xMnxPt3 system resulting from alloying two ferromagnetic CoPt3 and MnPt3 compounds have been investigated. Structural and microstructural characterizations added to thorough preparation conditions yield well-ordered samples having the L1(2) structure, large ordered domains, and a long-range order parameter close to its maximum value. Magnetic properties including ac and dc susceptibilities, zero-field-cooled and field-cooled magnetizations as a function of temperature and field (0-35 T) have been investigated. The resulting magnetic phase diagram displays, below a ferromagnetic region spreading over the whole (0 less than or equal to x less than or equal to 1) concentration range, a reentrant spin-glass phase for 0