The investigation of the Tl–Sr–Co–O system has allowed a new form of 1201-type cobaltite to be synthesized. The HREM study of this oxide, Tl0.4Sr2.5Co1.1O5 − δ (δ ≈ 0.20) shows that it consists of an association, in the same matrix, of two phasoïds, the tetragonal oxygen stoichiometric 1201-structure Tl0.4Sr2.5Co1.1O5 (a ≈ a1201, c ≈ c1201) and a new ordered oxygen deficient 1201 Tl0.4Sr2.5Co1.1O4.5 (a ≈ 2a1201, b ≈ a1201c ≈ 2c1201). The latter is described as the intergrowth of double rock salt layers [Sr1.5Tl0.4Co0.1O2]∞ with single ordered oxygen deficient perovskite layers [SrCoO2.5]∞ similar to YBaFeCuO5. The magnetic and transport properties of this Sr-rich oxide differ fundamentally from the 1201 cobaltite TlSr2CoO5, by the absence of a metal to insulator transition as a function of temperature. The as-synthesized sample exhibits a semiconducting and paramagnetic behavior down to 50 K, with θp = −160 K indicative of antiferromagnetic correlations. The oxygen pressure annealed sample shows a much lower resistivity, T independent down to 200 K, and weak ferromagnetism in agreement with its θp value (+50 K). These properties and thermopower measurements are interpreted on the basis of Co4+ carriers in an intermediate spins Co3+ matrix.