In polyfluorenes it is generally accepted that (photo) degradation leads to fluorenone type defects that accept the excitation energy and emit green-to-yellow light with rather low efficiency. Although initial spectroscopic studies suggest the same to hold true for ladder-type poly(para-phenylene)s (LPPPs), kinetic studies of the degradation process are not compatible with the established mechanism. In general, the observed green emission can be caused by the introduction of carbonyl groups; however, only if associated with an additional disruption of the backbone rigidity and hence planarity of the entire p-system. This is clearly shown by comparison with synthesized model compounds, which are bearing the fluorenone motif yet possess very different optical properties as compared to the defects, which are actually formed. Degradation can be caused by solvent specific, yet substrate nonspecific aromatic formylation but mainly originates from reaction with in-situ generated singlet oxygen, both in solution as well as in thin films. Time-dependent photoluminescence measurements on thin films show that green emission is enhanced by energy transfer from intact molecules to defect centers.