The　first　examples　of　alkali　metal　selenite　sulfates,　namely,　Na8(SeO3)(SO4)3　(1),　Na2(H2SeO3)(SO4)　(2),　and　K4(H2SeO3)(HSO4)2(SO4)　(3),　were　successfully　synthesized　by　hydrothermal　reactions.　Their　structures　display　three　different　zero-dimensional　configurations　composed　of　isolated　sulfate　tetrahedra　and　selenite　groups　separated　by　alkali　metals.　Na8(SeO3)(SO4)3　(1)　features　a　noncentrosymmetric　structure,　while　Na2(H2SeO3)(SO4)　(2)　and　K4(H2SeO3)(HSO4)2(SO4)　(3)　are　centrosymmetric.　Powder　second-harmonic-generation　measurements　revealed　that　Na8(SeO3)(SO4)3　(1)　shows　a　phase-matchable　SHG　intensity　about　1.2　times　that　of　KDP.　UV-vis-NIR　diffuse　reflectance　spectroscopic　analysis　indicated　that　Na8(SeO3)(SO4)3　(1)　has　a　short　UV　cutoff　edge　and　a　large　optical　band　gap,　which　makes　it　a　possible　UV　nonlinear　optical　material.　Theoretical　calculations　revealed　that　the　birefringence　of　Na8(SeO3)(SO4)3　(1)　is　0.041　at　532　nm,　which　is　suitable　for　phase-matching　condition.　This　work　provides　a　good　experimental　foundation　for　the　exploration　of　new　UV　nonlinear　crystals　in　an　alkali　metal　selenite　sulfate　system.　©　2024　American　Chemical　Society.