Compositions are provided, derived from chemical treatment of carbonaceous fiber felt forms or other carbonaceous substrates, that exhibit both high efficiency and capacity for mercury capture from flue gases and the like. These compositions are superior to known compositions, particularly with regard to their performance under severely deactivating conditions. The compositions result from the pyrolysis of certain specific sulfur bearing compounds while in contact with activated carbon powders, activated carbon fibers, or with other carbonized forms. Precursor compounds to these novel mercury-binding compositions contain structural moieties that may be represented as: where X, Y are individual atoms or are functional groups consisting of several atoms, such that the atom bonding to sulfur is not hydrogen. X and Y can be linked in cyclic structures, either directly or through other atoms.