Ternary Zintl phase materials of the formula K, l n, X, , where X=As or Sb, have been prepared following a hightemperature procedure. The crystal structure of K, I n, As, has been determined at low temperature and consists of [In,As:-], sheets of ca. 5 A thickness insulated by a layer of potassium ions of ca. 3 A thickness. The crystal structure of K41n,Sbs has been determined at room temperature and is isomorphous with the K, l n, Sb, compound. For K,I~,As,, a= 14.323(3) A, 6=7.106(2) A, c= 15.795(3) A, p=90.30(2)", space group P ~, / c , ~= 4 , p,=4.44 g cm-3 at T= lOO(5) K, R= 0.035 for 2688 unique observed reflections. For K, l n, Sb, , a= 15.282(5) A, b= 7.544(1) A, c= 16.788(3) A, /?=90.52(2)", space group P2,/c, Z=4, p,=4.62 g cmP3 at T=299 K, R=0.042 for 3001 unique observed reflections.