Abstract—We have determined the dimerization and monomerization cross sections of the ThyThy (cyclobutyl dimer of thymine and thymine) and the CytThy (cyclobutyl dimer of cytosine and thymine) dimers inEscherichia coti[3H]‐DNA ([3H]‐thymine labeled DNA) at five wavelengths in the range 240–300 nm. It may be concluded from the dimerization action spectra for the two dimers that the excitation of Thy (thymine) is mainly responsible for the photochemical dimerization reaction in both cases. The calculated quantum yields of dimerization and monomerization are also presented in this paper and several questions, raised by the results obtained at 300 nm, are di