The inherently oxygen-deficient compounds Ln(26)O(27)square(BO3)(8) (Ln = La, Nd) react with water vapor leading to Ln(26)O(26)(OH)(2)(BO3)(8) phases, and this reaction is reversible. The crystal structure of Nd26O27 square(BO3)(8) has been determined from single-crystal data (space group P1 with a = 6.7643(10) angstrom, b = 12.663(2) angstrom, c = 14.271 (2) angstrom, alpha = 90.553(8)degrees, beta = 99.778(10)degrees, and gamma = 90.511(9)degrees). It is a triclinic distorted version of the monoclinic structure of (LaO27)-O-26 square(BO3)(8). The Ln(26)O(26)(OH)(2)(BO3)(8) phases both crystallize in the monoclinic system (space group P211c with a = 6.7445(4) angstrom, b 12.6177(9) angstrom, c = 14.4947(10) angstrom, and beta = 100.168(7)degrees for Nd26O26(OH)(2)(BO3)(8) and a = 6.9130(15) angstrom, b = 12.896(3) angstrom, c = 14.792(4) angstrom, beta = 99.698(16)degrees for La26O26(OH)(2)(BO3)(8)), and their crystal structure has been determined from single-crystal data, showing that the hydroxyl groups are localized mainly,on one of the oxygen sites at room temperature (RT). For the Nd phases, the change in crystal system can result from two different phenomena depending on the atmosphere, either a phase transformation corresponding to a water uptake under wet conditions (triclinic Nd26O2 square(BO3)(8) at RT -> monoclinic Nd26O26(OH)(2)(BO3)(8)) or a phase transition at similar to 300 degrees C for the anhydrous phase under dry conditions (triclinic Nd26O27 square](BO3)(8) at RT -> monoclinic Nd26O27 square(BO3)(8) at T > 300 degrees C). For Nd26O26(OH)(2)(BO3)(8), the conductivity measured under wet conditions at 300 degrees C is sigma(300 degrees C) approximate to 0.5 x 10(-5) S cm(-1). Due to the dehydration process, the proton contribution to the total conductivity of the Nd phase is no longer observed above 500 degrees C whereas it was still clearly visible at 600 degrees C for the La phase.