A novel europium(III) membrane luminescence sensor based on a tridentate bis(phosphinic amide)-phosphine oxide, PhPO(C6H4POPhN(CH(CH3)2)2)2 (1), is described. The new luminescent complex, [Eu(1)2]Cl32, which is formed between europium(III) and ligand 1 and has a 1 : 2 stoichiometry, has been evaluated in solution. It has the excellent spectroscopic and chemical characteristics that make it appropriate for sensing film applications. All the parameters (polymer, plasticizer, ligand and ionic additive) that can affect the sensitivity and selectivity of the membrane sensor and instrumental conditions have been carefully optimized. The best sensing response (λexc = 229.04 nm, λem = 616.02 nm) was observed for 33.4 : 65.1 : 1.5 (%, w/w) PVC : DOS : 1. The sensing film shows a good response time (10 min) and a very good selectivity toward europium(III) with respect to other lanthanides(III) ions, such as La, Sm, Tb and Yb. The newly-developed sensing film has a linear range from 1.6 × 10−7 to 5.0 × 10−6 mol L−1 for Eu ions with a very low detection limit (4.8 × 10−8 mol L−1) and good sensitivity (9.41 × 10−7 a.u. mol−1 L−1) to europium. Complexes of [Eu(1)2]Cl3 (2) and [Eu(1)]Cl3 (4) were isolated by mixing ligand 1 with Eu(Cl3)·6H2O in acetonitrile at room temperature in ligand : metal molar ratios of 1 : 2 and 1 : 1, respectively. The 1 : 1 derivative is the product of thermodynamic control when a molar ratio of ligand to europium salt of 1 : 1 is used. The new compounds have been characterized in both the solid form (IR, MS-TOF, elemental analysis, TGA and X-ray diffraction) and in solution (multinuclear magnetic resonance). In both europium complexes, the ligand acts as a tridentate chelate. Thermogravimetric (TG) studies demonstrated that neither complex 2 or 4 possess any water molecules directly bound to the lanthanide metal, which corroborates the X-ray structure. The investigation of the solution behaviour of the Y(III) complexes with pulsed gradient spin-echo (PGSE) NMR diffusion measurements showed that average structures with 1 : 1 and 1 : 2 stoichiometries are retained in acetonitrile solutions.