The effects of the lanthanoid cations (Ln(3+)) on the first electronic transition ((A) over tilde <- (X) over tilde) of liquid water were studied from the attenuated total reflection far-ultraviolet (ATR-FUV) spectra of trivalent Ln(3+) electrolyte solutions (1 M), except Pm3+. The (A) over tilde <- (X) over tilde transition energies of the Ln(3+) electrolyte solutions show a distinct tetrad in their dependence on the number of 4f electrons of the Ln(3+) cations. For the half occupation period of the 4f electrons, the (A) over tilde <- (X) over tilde transition energies decrease from La3+ (4f(0), 8.0375 eV) to Nd3+ (4f(3), 8.0277 eV) and increase from Sm3+ (4f(5), 8.0279 eV) to Gd3+ (4f(7), 8.0374 eV). For the complete occupation period, there are two local minima at Dy3+ (4f(9), 8.0349 eV) and Yb3+ (4f(13), 8.0355 eV). The (A) over tilde <- (X) over tilde transition energies of the tetrad nodes (La3+, Gd3+, Ho3+ (4f(10)), and Lu3+ (4f(14))) increase slightly, as the nuclear charge increases in accordance with the hydration energies of the Ln(3+) cations. The energy difference (Delta E) between the (A) over tilde <- (X) over tilde transition energies and the line between La3+ and Lu3+ is largest at Nd3+ (80.5 cm(-1)) for the half occupation period and at Dy3+ (26.1 cm(-1)) and Yb3+ (24.5 cm(-1)) for the complete occupation period. The order of magnitude of Delta E is comparable to the ligand field splitting (LFS) of the ground state multiplets of Ln(3+) complexes. The observed tetrad trend of the (A) over tilde <- (X) over tilde transition energies of the Ln(3+) electrolyte solutions across the 4f period reflects the hydration energies of the Ln(3+) cations and the LFS induced by water ligands.