Photochemical profiles of beta-bond dissociation in highly excited triplet states (T-n) of biphenyl derivatives having C-O bonds were investigated in solution, using stepwise laser photolysis techniques. The lowest triplet states (T-1) were produced by triplet sensitization of acetone (Ac) upon 308-nm laser photolysis. The molar absorption coefficients of the T-1 states were determined using triplet sensitization techniques. Any photochemical reactions were absent in the T-1 states. Upon 355-nm laser flash photolysis of the T-1 states, they underwent fragmentation, because of homolysis of the C-O bond in the T-n states from the observations of the transient absorption of the corresponding radicals. The quantum yields (Phi(dec)) for the decomposition of the T-1 states upon the second 355-nm laser excitation were determined. Based on the Phi(dec) values and the bond dissociation energies (BDEs) for the C-O bond fission, the state energies (E-RT) of the reactive highly excited triplet states (T-R) were determined. It was revealed that (i) the Phi(dec) was related to the energy difference (Delta E) between the BDE and the E-RT, and (ii) the rate (k(dis)) of beta-cleavage in the T-R state was formulated as being simply proportional to Delta E. The reaction mechanism for beta-bond cleavage in the T-R states was discussed.