Raman spectrum of burgessite, Co2(H2O)4[AsO3OH]2· H2O, was studied, interpreted and compared with its infrared spectrum. The stretching and bending vibrations of (AsO3) and As-OH units, as well as the stretching, bending and libration modes of water molecules and hydroxyl ions were assigned. The range of OH···O hydrogen bond lengths was inferred from the Raman and infrared spectra of burgessite. The presence of (AsO3OH)2− units in the crystal structure of burgessite was proved, which is in agreement with its recently solved crystal structure. Raman and infrared spectra of erythrite inferred from the RRUFF database are used for comparison. Copyright © 2010 John Wiley & Sons, Ltd.