A　new　selenite　compound　CaCu(SeO3)2　was　obtained　by　a　hydrothermal　method.　This　compound　crystallizes　in　the　monoclinic　system　of　space　group　P21/c,　featuring　a　typical　spin-dimer　structure.　The　results　of　magnetic　and　heat　capacity　measurements　confirm　that　this　compound　has　a　nonmagnetic　singlet　ground　state,　while　a　fit　of　magnetic　susceptibility　using　the　spin-dimer　model　suggests　a　large　spin　gap　of　∼329　​K　in　the　system.　Although　CaCu(SeO3)2　and　HgCu(SeO3)2　have　a　similar　spin-dimer　structure　built　by　[Cu2O8]　dimers　and　SeO3　groups,　the　spin　gap　of　CaCu(SeO3)2　is　much　larger　than　that　of　HgCu(SeO3)2.　Such　different　spin-gap　behaviors　may　be　due　to　a　change　on　the　bonding　geometry　of　spin　dimers　with　a　substitution　of　Ca2+　ions　for　Hg2+　ions,　leading　to　stronger　intradimer　and　weaker　interdimer　interaction　exchanges　in　CaCu(SeO3)2.　Also,　the　solid-state　UV–vis　diffuse　reflectance　spectrum　shows　an　optical　band　gap　of　2.71　​eV　in　the　system.　©　2022　Elsevier　Inc.