The stoichiometry range for Sr-doped La-123/336 solid solutions has been determined by means a phase diagram study at ∼950°C. The solid solutions have general formula La1.5-xBa1.5+x-ySryCu3Oz and form over the entire range 0≤x≤0.5 and 0≤y≤0.5 apart from compositions with high x and y. The solid solutions are tetragonal under all conditions used; lattice parameters vary linearly with x and y. Tc passes through a maximum of 65 K at x∼0.5, y∼0.3, which is ∼ 10 K higher th the previous maximum reported for this system. Oxygen content determinations by TGA indicate two mechanisms for increasing the hole concentration: with increasing x, the substitution La3+⇔Ba2++h+ occurs, whereas with increasing y, the substitution mechanism is Ba⇔Sr+mO2-+2mh+. Rietveld refinement of powder XRD data shows that Sr substitutes preferentially onto Ba sites, in contrast to Ca substitution where Ca occupies La sites.