Two interesting mixed Eu³⁺/Y³⁺ lanthanide metal–organic frameworks, formulated as [EuY₂(H₂O)₆(IN)₃(ox)₃]·H₂O (1) and Eu_3.5Y_4.5(μ₂-OH)₈(IN)₈(ox)₄ (2) (HIN = isonicotinic acid; H₂ox = oxalic acid), have been hydrothermally synthesized under the same initial reaction conditions except pH values. Such mixed Ln MOFs which contain two types of ligands are quite novel, and the pH value largely affects the topologies and Eu/Y ratios of the two compounds. Compound 1 exhibits a two-dimensional (2D) layered structure which is built up of [Ln₂IN₂]⁴⁺ units connected by oxalate ligands with a Eu/Y ratio of about 1 : 2, while in 2, the 2D Ln-ox metal–organic layers are connected by IN⁻ ligands to give rise to a 3D framework with a Eu/Y ratio of about 1 : 1.3. Moreover, the photoluminescence properties of the two compounds indicate that fluorescence intensity of 1 is much greater than that of 2 due to the different levels of energy transfer and coordination environment.