Trisilanol polyhedral oligomeric silsesquioxane (POSS) derivatives have recently been reported as a new class of amphiphilic molecules that form stable Langmuir monolayers at the air/water interface and exhibit interesting phase transitions and aggregation behavior. In this letter, surface light scattering (SLS) has been used to probe the dilational viscoelastic behavior of trisilanolisobutyl-POSS in the monolayer regime of its surface pressure-area per molecule isotherm at 22.5 degrees C. Results showed that the static dilational elastic modulus is in excellent agreement with the dynamic dilational elastic modulus in the monolayer regime over the limited wavevector range of 404.3-512.6 cm(-1). The films possessed a moderate maximum dynamic dilational elastic modulus (similar to 50 mN.m(-1)) prior to film collapse. Moreover, the films exhibited small surface viscosities such that the films arc almost purely elastic over the frequency range studied throughout the monolayer regime.