A series of Ln(III)-substituted polyoxometalates (POMs) having molecular formula Cs x K 15−x [(AsW 9 O 33 ) 3 Ln 2 (H 2 O) 7 W 4 O 9 ]•yH 2 O were synthesized by using dilacunary arsenotungstate [As 2 W 19 O 67 (H 2 O)] 14− as a precursor with Ln(III) ions [Ln = Dy (1), Tb (2), Er (3), Ho (4), Yb (5), Eu ( 6), x = 6−7, y = 24−40]. The Ln-POM complexes comprise a fusion of three trivacant arsenotungstates (ATs) [α-AsW 9 O 33 ] 9− building units resembling an oxalis triangularis-like arrangement sealing a heterometallic [Ln 2 (H 2 O) 7 (W 4 O 9 )] 12+ core unit. Detailed magnetic studies disclose that complex 1 shows slow relaxation of magnetization at zero and 2000 Oe external dc fields due to its high axial anisotropy. An energy barrier of 56.48 K is obtained under the 2000 Oe applied dc field, whereas only the QTM process is favorable in the absence of dc field. On the other hand, complex 5 shows field-induced slow relaxation of magnetization with 5 and 6 K phenomenological energy barriers under 1500 and 2000 Oe applied dc fields. Furthermore, ab initio calculations confirm that high axial ground states get stabilized with minimum transverse anisotropy for complex 1 whereas easy plane anisotropy is observed for complex 5. It is also observed that the arrangement of the POM cluster provides intermediate axial symmetry (C 2v ) around the metal centers, stabilizing higher m j states along with high anisotropy for both Dy(III) and Yb(III) ions in complexes 1 and 5. Thus, both Dy(III) (oblate ion) and Yb(III) (prolate ion) Ln-POM clusters display slow magnetic relaxation, despite having different spatial distributions of 4f electronic density. The solid-state photoluminescence spectra of complexes 1−4 and 6 were investigated, which displays the characteristic emission of Ln(III) components based on 4f−4f transitions.