One new compound, ZrLaTa₃O₁₁ has been synthesized in the system ZrO₂–La₂O₃–Ta₂O₅, with an orthorhombic unit cell, a= 10.890(3)Å, b= 12.450(3)Å and c= 6.282(2)Å. Extensive ternary solid solutions are formed by five of the binary phases: La₂Zr₂O₇, La₃TaO₇, LaTaO₄, LaTa₃O₉ and LaTa₅O₁₄. Two main solid-solution mechanisms are in evidence: La + Ta ⇌ 2Zr and La + 3Zr ⇌ 3Ta. The solid-solution phase previously described as being based on Ta₂Zr₆O₁₇ has, instead, the ideal stoichiometry, TaZr_2.75O₈. A significant correction is made to the unit cell dimensions of La₃ TaO₇. The subsolidus phase diagram of the ZrO₂–La₂O₃–Ta₂O₅ system has been determined at 1500 °C. The electrical conductivity of a selection of the phases has been determined. Most are very poor semiconductors, but one, LaTa₃O₉, has a high conductivity, 10^–4Ω^–1 cm^–1 at 400 °C, which appears to be due to oxide ions. ZrLaTa₃O₁₁ by contrast is an excellent insulator with a resistivity 10⁶Ω cm at 800 °C.