Dilatational viscoelasticities of a nonionic surfactant monolayer are measured at the air/water interface using the excited-capillary-wave technique. The data are compared with the dif fusion-exchange model for surfactant adsorption. This model requires the knowledge of the equation of state for the monolayer. We have used the Frumkin equation of state. Although the measured elasticities show the same qualitative trends predicted by the model, their absolute values fall well below the calculated ones in the diffusion-dominated regime. In order to quantitatively fit the model to our data, rather high values of the diffusion coefficient must be used. However, we independently determine the diffusion coefficient for this system, via dynamic surface tension measurements using the relaxing pendent-drop technique, and our results agree with the expected bulk value within the experimental uncertainty. When the diffusion-model parameters are calculated numerically without assuming any form of equation of state to relate gamma, Gamma, and c, the model fits the data with the bulk diffusion coefficient value within experimental uncertainties. This suggests that the current monolayer equations of state are not able to accurately describe the response of a monolayer to small periodic dilatations around equilibrium.