In this study, two biodegradable polymers, poly(trimethylene carbonate) (PTMC) and poly(L-lactide) (PLLA) along with a series of PTMC/PLLA blends, were used as spreading materials to form LB monolayers at the air/water interface to study hydrolytic reaction kinetics of the monolayers with the Langmuir film balance technique. The pi-A isotherms of each homopolymer and their blends showed that blends of PTMC and PLLA were miscible on the neutral subphase (pH 7.4), whereas there was evidence of phase separation on the basic subphase (pH 10.7). The hydrolysis behavior of each homopolymer was investigated at these two different pH conditions. The PTMC monolayer showed faster hydrolysis on the neutral subphase (pH 7.4) than on the basic subphase (pH 10.7). However, in the case of the PLLA monolayer, the hydrolysis on the basic subphase is faster than that on the neutral subphase. On the basis of this result, hydrolysis mechanisms of PTMC and PLLA, considering a general hydrolysis mechanism and their stereo structures, are proposed. The hydrolysis rates of blends of PTMC and PLLA were much faster than that of each homopolymer on the basic subphase (pH 10.7). This result, which can be explained by a "dilution effect", was supported by the structure based mechanism proposed here.