We have studied the evolution of the T1 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 T1 state emission (phosphorescence) is easier to detect. For both, the platinum-containing polymer series and for the all-organic polymers, we find the T1 state to be at a constant separation of 0.7±0.1 eV below the singlet S1 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 S1–T1 gap can be increased by confining the S1 state in oligomers and monomers.