Short time adsorption and desorption behaviors of two alkyl phthalate esters, namely diethyl phthalate (DEP) and dibutyl phthalate (DBP), on silicon wafers exposed under various ambient concentrations were experimentally and theoretically investigated. The results showed that the surface density of DBP was significantly affected by both the length of exposure time and its ambient concentration, whereas that of DEP was only affected by its ambient concentration within the tested periods between 60 and 240 min. The determination of rate parameters for adsorption and desorption showed that the rate constants of DEP were always larger than those of DBP. Also, the sticking coefficient of DEP was larger during the initial adsorption stage due to its relatively lower molecular weight as compared to DBP. The value of the sticking coefficient for DEP, however, decreased much faster such that the value eventually became smaller than that for DBP. Therefore, for silicon wafers experiencing a short exposure time, organic compounds with lower molecular weights may be a more important source of airborne molecular contamination than those with higher molecular weights.