Cation ordering/disordering features in perovskite-like oxides LnBaMn2O6−δ where Ln = Pr, Nd, Sm were studied by means of a combination of thermogravimetry and X-ray powder diffraction techniques in a wide range of temperatures. Variation in lanthanide size was shown to be an effective route for the shift in temperature of ordering and disordering. As a result, it was found coexistence of three different phases which can be obtained separately: disordered cubic Ln0.5Ba0.5MnO3–δ, and ordered kinds of LnBaMn2O6−δ having δ ≈ 0 or 1.0. The respective ambient conditions around stability boundaries for the whole studied phases were successfully determined. Additionally the main reasons for inducing ordering as well as disordering processes were clarified by the use of in-situ XRD and DSC methods. A combination of the experimentally obtained data with the results of limited studies reported in literature allows one to organize the available information about double manganites giving some explanation of ordering processes as well as appropriate conditions used to obtain oxides with desired ordering degree.