A general technique for the preparation of complex Tl-Sr-La cobaltites, via the interaction of a La2-xSrxCoO4-y precursor with Tl2O3 in a sealed gold ampoule was elaborated. It allowed obtaining the compound Tl0.64Sr1.5La0.5CoO5-epsilon, whose bulk and surface compositions were investigated by EPMA and ESCA, respectively. This new phase is isostructural with the 1201 member of the thallium cuprate series with tetragonal unit-cell parameters a = 3.781 Angstrom, = 8.697 Angstrom. The specific features of the synthesis mechanism that result in kinetic limitations of the purity of the reaction product were analyzed. Various possibilities for accommodating the large observed thallium deficiency were considered regarding the complex structure of the oxide.