Three new luminescent cyclometalated Pt(II) complexes, [Pt(L)Cl] (1), [Pt-2(L-)(2)] (2), and [Pt(L)(PPh3)]ClO4 (3 center dot ClO4) (where HL = 2-phenyl-6-(1H-pyrazol-3-yl)-pyridine), were synthesized and characterized by X-ray crystallography. HL represents a new class of C,N,N-pyrazolyl cyclometalating ligands containing a C-phenyl, a N-pyridyl, and a N-pyrazolyl donor moiety, as well as a 1-pyrazolyl-NH, that can also be available for metal coordination and other chemical interactions. Complex 1 possesses intense intraligand transitions at 275-375 nm and moderately intense metal-to-ligand charge transfer ((MLCT)-M-1) (d pi(Pt) -> pi*(L)) transition at 380-410 nm. The room temperature solid-state emission lambda(max) of 1 occurs at 580 nm and is attributable to the (MMLCT)-M-3 (d sigma*(Pt) -> pi*(L)) transition. It also displays strong phosphorescence in acetonitrile solutions at room temperature with an emission lambda(max) at 514 nm, which can be tentatively assigned to the (MLCT)-M-3 (pi*(L) -> d pi(Pt)) transition. Complex 1 can be deprotonated in organic solvents to yield a cycloplatinated dimer 2, which shows a relatively high room-temperature luminescent quantum yield of 0.59 in DMF (lambda(max) = 509 nm). Substitution of the ancillary chloro-ligand in 1 by triphenylphosphine yields 3, which also possesses a good room-temperature luminescent quantum yield of 0.52 in DMF (lambda(max) = 504 nm) and a better solubility in water. Complex 3 is synthesized to demonstrate the pH dependence of luminescent properties of this C,N,N-pyrazolyl cyclometalated Pt(II) system. Such a pH response is ascribable to the protonation/deprotonation of the 1-pyrazolyl-NH on the C,N,N-pyrazolyl cyclometalating ligand. The pK(a) of the 1-pyrazolyl-NH in 3, measured in 1:2 (v/v) aqueous DMF solutions, is approximately 4.0.