The mechanism underlying 248-nm laser ablation of liquid benzene derivatives (alkyl benzenes, benzyl chloride, benzyl alcohol) was revealed by means of transient absorption spectroscopy. One characteristic in the present liquid system is that the ablation threshold can be correlated not to the boiling point at all but to the photochemical reactivity of beta-bond cleavage. In the spectroscopic measurement, the benzyl-radical formation was confirmed upon ablation, and its Concentration was quantitatively evaluated at the threshold. The obtained value was similar to 0.05 M, which was almost common to all of the examined Liquids. The result means that the present liquids, whose macroscopic physical properties such as surface tension do not differ too much from each other because of the analogous molecular structure, undergo the ablation when the radical concentration reaches the critical value of 0.05 M. This is the first demonstration for bridging a gap between microscopic photochemical process and macroscopic morphological change. The ablation behavior can be well interpreted in terms of the photochemical volume explosion.