The first mixed-valent boroseleniteselenate B2Se3O10 was obtained as a colorless, hygroscopic compound from the reaction of boric acid (H3BO3) in concentrated selenic acid (H2SeO4) at 265 degrees C. H2SeO4 can be replaced by appropriate amounts of SeO2 and H2O2. The crystal structure was determined from single-crystal data (P2 (1)/c, Z = 4, a = 4.3466(2) angstrom, b = 7.0237(4) angstrom, c = 22.1460(9) angstrom, beta = 94.922(2)degrees, R1 = 0.036, wR2 = 0.096). It represents a new structure type that is characterized by a 3D net of BO4 tetrahedra, (SeO4)-O-VI tetrahedra, and trigonal-pyramidal (SeO3)-O-IV in a ratio of 2:1:2. Se-77 magic-angle-spinning NMR investigations confirm the mixed-valent character because the chemical shifts are found in the typical regions, i.e., 1278 and 1202 ppm for Se-IV and 972 ppm for Se-VI. The vibrational spectra show the typical modes according to the present polyhedra. In addition, NMR and vibrational spectra of the closely related B2Se2O7 are presented.