Formation of pyrimidine dimers in DNA is a major initiating event in the induction of skin cancer. Model experiments suggest that, upon absorption of UVA, one type of dimers induced by UVB, the pyrimidine (6‐4) pyrimidone photoproducts, photosensitizes the formation of mutagenic cyclobutane pyrimidine dimers by triplet ‐triplet energy transfer (TTET). We investigated whether this photoreaction actually took place when 64PP were located within a DNA duplex rather than added as external sensitizers like in available data. Our results show that this process is not detectable in DNA and double‐stranded oligonucleotides exposed to a combination of UVB and UVA. TTET could only be observed, as a very minor photoreaction, in a short single‐stranded oligonucleotide bearing a 64PP. It may be concluded that 64PP‐mediated TTET does not significantly contribute to UV‐induced DNA damage. In contrast, the photoisomerization of 64PP into their Dewar valence isomers is very efficient.