Single crystals of lanthanum oxyhydroxide (LaOOH) were grown using an electrochemical method. Crystals having a maximum size of 7.0 mm in length, 1.5 mm in width, and 0.7 mm in thickness were grown. LaOOH crystallizes in a monoclinic structure of the space group P2(1)/m with a lattice parameter of a = 0.4417 nm, b = 0.3926 nm, c = 0.6573 nm, and beta = 112.32 degrees. LaOOH crystals completely changed to lanthanum trihydroxide La(OH)(3) within 48 h in ambient atmosphere by water absorption. Maintenance at 390 degrees C by heat treatment was effective to maintain the structure of LaOOH for at least a week. (c) 2007 Elsevier B.V. All rights reserved.