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Two open issues on the measurement of the dilational modulus (<i>E</i>) for an adsorbed protein film during the adsorption process have been unacknowledged: how <i>E</i> varies during the adsorption and the length of time needed to attain a stable <i>E</i> value. A new approach for detecting the <i>E</i> variation from a clean air–water interface to saturated film and estimating the time needed to reach a saturated state was proposed. A pendant bubble tensiometer was utilized for measuring the relaxations of surface tension (ST) and surface area (SA), and the <i>E</i> was evaluated from the relaxation data of minute distinct perturbances. The data showed a clear variation in <i>E</i> during the BSA adsorption: <i>E</i> sharply decreased to a minimum at the early stage of BSA adsorption; then, it rose from this minimum and oscillated for a while before reaching an <i>E</i> corresponding to a saturated BSA film after a significant duration. The adsorbed BSA film took ~35 h to reach its saturated state, which was much longer than the reported lifetime of the adsorbed film in the literature. A rapid surface perturbation (forced bubble expansion/compression) could change the <i>E</i>, causing a significant drop in <i>E</i> followed by a slow increase to the original stable value.