We report the preparation and the electrochemical and spectroscopic properties of the [Ru(bpy)(2)(ABME)](3+) and [Os(bpy)(2)(ABMe)](3+) complexes, where bpy is 2,2’-bipyridine, AB is bis-chelating 2,2’:3’,2 ":6 ",2triple prime quaterpyridine (which is composed of a bpy-type fragment, A, coordinated to the metal center, and a free bpy-type fragment, B), and Me indicates methylation at a pyridyl N of site B. The electrochemical and photophysical properties (luminescence spectra, emission lifetimes, quantum yields) of the title complexes have been compared with those of the parent species [Ru(bpy)(2)(AB)](2+) and [Os(bpy)(2)(AB)](2+) in which the pendant bpy site B of the AB ligand is not methylated. In the methylated complexes a charge-separated energy level is present which involves the oxidized metal center and the reduced methylated center. For energetic reasons, this level cannot be reached from the luminescent metal-to-ligand charge-transfer level and no quenching of the Ru- and Os-based luminescence takes place : the spectroscopic changes observed with respect to the related unmethylated complexes may be understood in terms of perturbation effects. The luminescence properties of the investigated complexes are affected by acidity of the solvent and HCl titration in acetonitrile:water (1:1) solvent for [Ru(bpy)(2)(AB)](2+) and [Ru(bpy)(2)(ABMe)](3+) has been employed to investigate acid-base equilibria at B site. In the 4 < pH < 1.5 interval, a deprotonation-protonation equilibrium is observed for [Ru(bpy)(2)(ABH)](3+), whose excited and ground states exhibit a similar poorly basic character, being pK(a)* = 2.28, pK(a) similar to 1.9, and Delta pK(a) similar to 0.4. The luminescence properties of [Ru(bpy)(2)(ABMe)](3+) do not change at pH > 2 : the methyl group protects the free N center of B site from proton access in this pH range. In more acidic solvent, pH < 2, the luminescence of [Ru(bpy)(2)(ABMe)](3+) is quenched {and so does at pH < 1.5 that of [Ru(bpy)(2)(ABH)](3+)}, because of a protonation process occurring in nonequilibrium conditions with a second order rate constant k(pr) = 5.3 x 10(7) M(-1) s(-1). A discussion is given on the nature of the process occurring at pH < 2.