We have studied the evolution of the T-1 triplet excited state in an extensive series of phenylene ethynylene polymers and monomers with platinum atoms in the polymer backbone and in an analogous series of all-organic polymers with the platinum(II) tributylphosphonium complex replaced by phenylene. The inclusion of platinum increases spin-orbit coupling so T-1 state emission (phosphorescence) is easier to detect. For both, the platinum-containing polymer series and for the all-organic polymers, we find the T-1 state to be at a constant separation of 0.7+/-0.1 eV below the singlet S-1 state. It is not possible to change this singlet-triplet splitting by altering the size or the charge-transfer character of the polymer repeat unit or by changing the electron delocalization along the polymer backbone. The S-1-T-1 gap can be increased by confining the S-1 state in oligomers and monomers.