A CIDEP study on the triplet mechanism (TM) was carried out on three systems, zinc tetraphenylporphyrin (TPP) and p-benzoquinone (p-BQ) in alcohols, pyrazine/2-propanol, and maleic anhydride/2-propanol, in three different microwave frequency regions, i.e. X-(9.5 GHz), S-(3.0 GHz), and L-(1.5 GHz) bands. The CIDEP spectra involving TM contributions drastically change depending on the microwave (MW) frequency or the magnetic field. Tn the ZnTPP and p-BQ systems the TM contribution increases with a decrease of the MW frequency, whereas in the other two systems the TM contribution increases with an increase of the MW frequency. The results can be explained by the frequency dependence of the initial spin polarization produced by the TM which is strongly influenced by the rotational correlation time, the zero field splitting, and the reaction rate of the triplet molecule. The experimental results are compared with the predictions of the Atkins-Evans theory. Reasonable agreements between the experimental results and the theoretical predictions are obtained.