The hydrolysis reaction of L-alpha- dipalmitoylphosphatidylcholine (L-DPPC) catalyzed by cabbage phospholipase D (PLD) at the air/water interface has been studied by Brewster angle microscopy (BAM) and film balance technique. The curves of surface pressure versus time show that the hydrolysis reaction depends on the initial state of the lipid monolayer. The BAM images display that the hydrolysis reaction preferentially occurs in the liquid-expanded phase where PLD has a maximum activity. In the coexistence region of liquid-expanded and liquid-condensed phases, a "lotus-like" domain of the monolayer was observed, indicating that the hydrolysis product 1, 2-dipalmitoylphosphatidic acid (DPPA) created a new phase, which inhibited the reaction rate of hydrolysis.