The ruthenium(II) complex salts [Ru(NH 3 ) 5 L][PF 6 ] 2 [L = coumarin 510 (C-510) 1 or coumarin 523 (C-523) 2] have been synthesized and fully characterized. Their Ru III analogues [Ru(NH 3 ) 5 L][PF 6 ] 3 (L = C-510 3 or C-523 4) have been prepared by oxidation of 1 or 2, respectively, with AgO 2 CCF 3 . Electronic spectroscopy and cyclic voltammetry have shown that the ruthenium centres exert an electron-withdrawing influence on the dyes, which is greater for C-523. The intense, low-energy intramolecular charge-transfer (i.c.t.) absorptions of the C-523 complexes displayed marked solvatochromism, whilst those of the C-510 complexes showed only very slight solvent dependence. Measurements of the first hyper-polarizability, β, by using the hyper-Rayleigh scattering technique at 1064 nm yielded large values in the range (120–420) × 10 -30 esu, the largest being for [Ru(NH 3 ) 5 (C-523)][PF 6 ] 2 2. These were enhanced by resonance via the i.c.t. excitations. Correction for resonance effects by using the two-level model afforded static hyper-polarizabilities (β 0 ) in the range 35–49 × 10 -30 esu, with [Ru(NH 3 ) 5 (C-523)][PF 6 ] 3 4 having the largest. The β 0 value for 4 was larger than that determined for C-523 via electric-field-induced second harmonic generation, indicating that complexation of a {Ru(NH 3 ) 5 } 3+ moiety enhanced the hyper-polarizability of this dye. This has been ascribed to the electron-withdrawing effect of the Ru III centre.