The synthesis, crystal structure, and characterization of a non-centrosymmetric oxide, TeSeO4, are reported. The material was synthesized by combining TeO2 and SeO2 in a quartz tube and heating at 370 degreesC. TeSeO4 has a three-dimensional structure containing [SeO3/2](+) cations linked to [TeO5/2](-) anions. Both the Se4+ and the Te4+ atoms are in asymmetric environments owing to their nonbonded electron pair. The material is SHG active, with a SHG intensity of 400 times SiO2. Crystal data: monoclinic, space group la (No. 9, cell choice 3), with a = 4.3568(8) Angstrom, b = 12.465(3) Angstrom, c = 6.7176(15) Angstrom, beta = 90.825(4)degrees, and Z = 4.