The two title trialkaline trioxoantimonates(III), tripotassium trioxoantimonate(III), K(3)[SbO(3)], (I), and tricaesium trioxoantimonate(III), Cs(3)[SbO(3)], (II), crystallize in the cubic Na(3)[AsS(3)] structure type in space group P2(1)3. The structures show discrete Psi-tetrahedral [SbO(3)](3-) anions with C(3v) point-group symmetry. The Sb-O distances are 1.923 (4) A in (I) and 1.928 (2) A in (II), and the O-Sb-O bond angles are 99.5 (2) degrees in (I) and 100.4 (1) degrees in (II).