Honokiol, 5,5'-diallyl-2,4'-dihydroxy- biphenyl, by comparison with its isomer magnolol, 5,5'-diallyl- 2,2'-dihydroxy- biphenyl, has been characterized by steady-state and time-resolved spectroscopy as well as (1)H NMR. Honokiol shows more complex pH dependence of absorption and fluorescence characteristics compared with magnolol. Honokiol possesses much weaker acidity than magnolol both in the ground and excited states. Its weak photoacidity is similar to that of 4-hydroxy- biphenyl or 4, 4'-dihydroxy- biphenyl rather than 2-hydroxy- biphenyl or 2, 2'-hydroxy- biphenyl. The electron effect and geometry configuration of substitution has been discussed.