The lanthanide(III) complexes formed with the tri‐ and tetraacetate derivatives of bis(aminomethyl)phosphinic acid, L1 and L2, respectively, have been studied by pH potentiometry, spectrophotometry and 1H and 17O NMR spectroscopy. L1 forms [Ln(L1)]–, [Ln(L1)2]4–, protonated [Ln(HL1)] and Ln(H2L1)]+, and [Ln(L1)(OH)]2– hydroxido complexes. Heptadentate L2 forms [Ln(L2)]2– and protonated [Ln(HL2)]– and [Ln(H2L2)] complexes in solution and it shows a strong propensity to form [Ln2(L2)]+ dinuclear complexes, which has not been observed previously. The stability constants (log KLnL) of the complexes increase in the order [Ln(L1)]– < [Ln(L2)]2– following the order of increasing number of acetate pendants attached to the bis(aminomethyl)phosphinic acid (BAP) backbone. Within the LnIII series, the log KLnL values increase from La3+ to Gd3+ and remain practically constant for the heavier lanthanides. Despite the lower basicity, the ligands that contain a phosphinate group generally form similar (L1) or more stable (L2) Ln3+ complexes than the structurally similar N‐benzylethylenediamine‐N,N′,N′‐triacetic acid (L3) and propylenediamine‐N,N,N′,N′‐tetraacetic acid (L4), respectively. This indicates that the hard phosphinate group may be coordinated to the Ln3+ ions in the complexes, whereas the larger negative charge of the BAP derivatives may also have an extra stabilizing effect. The kinetic inertness of [Ln(L1)] and [Ln(L2)] is lower than that of similar [Ln(EDTA)]– (EDTA = ethylenediamine‐N,N,N′,N′‐tetraacetic acid), but the rate constants that characterize the dissociation of [Ln(L2)]2– are at least two orders of magnitude lower than those obtained for [Ln(L4)]–. Variable‐temperature 17O transverse and longitudinal relaxation rates and NMR spectroscopic chemical shifts have been measured to assess the water exchange and rotational dynamics of [Gd(L2)]. The chemical shifts evidenced monohydration of the complex. The water exchange rate, kex298 = (2.7 ± 0.4) × 107 s–1 is about ten times higher than that of [Ln(DTPA)]2– (DTPA = diethylenetriamine‐N,N,N′,N″,N″‐pentaacetic acid). The rotational correlation time, τRO298 = 270 ± 30 ps, is long considering the small size of the chelate, which points to aggregation in aqueous solution, in accordance with the high value of the proton relaxivity measured.