Millions of tons of sodium borates are used annually by global industries in diverse applications important to modern society. The Na2O-B2O3-H2O phase diagram in the 0-100 degrees C temperature range contains 13 unique hydrated crystalline sodium borates, including five important industrial products. Structures were previously reported for each of these except for that having the highest boron content, known as sodium enneaborate, Na4B18O29 center dot 11H(2)O or 2Na(2)O center dot 9B(2)O(3 center dot)11H(2)O (1). Here we report the single-crystal structure of 1, revealing the structural formula Na-2[B8O11(OH)(4)]center dot B(OH)(3)center dot 2H(2)O, and describe some of its properties and relationships to other sodium borates. The Structure Of 1 features linear polyborate chains composed of the repeating [B8O11(OH)(4)](2-) fundamental building blocks with interstitial water and boric acid molecules integrated by extensive H bonding. Interstitial sodium cations occur in groups of four with interatomic distances of 3.7830(6) and 3.7932(8) angstrom. Upon heating, 1 initially becomes amorphous and then crystallizes as alpha-Na2B8O13 along with amorphous B2O3. Notalily, alpha-Na2B8O13 contains octaborate fundamental building blocks that are topologically equivalent to those in 1. Compound 1 Crystallizes fir the monoclinic space group P2(1)/n with a = 10.2130(8) angstrom, b = 12.940(1) angstrom, c = 12.457(1) angstrom, beta = 93.070(2)degrees V = 1644.0(2) angstrom(3), and Z = 2.