The emitting triplet state of cyclometalated Pt(thpy)(CO)(CI) monomers ((thpy)(-) = 2-(2'-thienylpyridinate), frequently also abbreviated as (2-thpy)(-)) is investigated at T = 1.2 K (typically) by use of the complementary methods of high-resolution optical spectroscopy and of optically detected magnetic resonance (ODMR) spectroscopy. Such a complimentary investigation is carried out for the first time for a Pt(II) compound. In solution, oligomer or short linear chain formation is also observed. However, the monomers can be investigated selectively, when they are dissolved in a relatively inert n-octane matrix (Shpol'skii matrix). This allows us to determine the energies of the T-1 triplet substates I, II, and III relative to the electronic ground state S-0(0), the zero-field splittings (ZFSs) of T-1, and emission decay time constants (I/II <----> 0, 18012.5 cm(-1); III <----> 0, 18016.3 cm(-1); DeltaE(I,II) = 0.05437 cm(-1) (1.631 GHz), DeltaE(I,III) = 3.8 cm(-1) (114 GHz); tau(I) 120 mus, tau(II) = 45 mus, tau(III) = 35 mus; spin-lattice relaxation time for the processes III --> I/II, tau(SLR) = 3.0 mus). The vibrational satellite structure observed in the emission of the T-1 state to the singlet ground state So is also discussed. Moreover, it is possible to estimate the intersystem crossing time from the excited singlet state S-1 at 22952 cm(-1) to the triplet state T-1 to similar to5 ps. The T-1 state is assigned as a thpy-ligand-centered (3)pipi* state with small metal-to-ligand charge-transfer (MLCT) admixtures. A comparison of Pt(thpy)(CO)(Cl) to a series of other organometallic Pt(II) compounds, such as heteroleptic Pt(ppy)(CO)(Cl) ((ppy)= = phenylpyridinate), Pt(dppy)(CO) ((dppy)(2-) = diphenylpyridinate), and Pt(i-biq)(CN)(2) (i-biq = 2,2'-bisisoquinoline) and homoleptic Pt(thpy)(2) and Pt(ppy)(2), is carried out. (The structures are shown in Figure 7.) Trends of photophysical properties are discussed. In particular, by chelation of two equal ligands the pattern of ZFS is strongly altered, resulting in a significant increase of the MLCT participation in the lowest triplet state of these organometallic compounds. This new observation represents an interesting further step concerning chemical tunability of photophysical properties.