A new interface model with zero thickness and intrinsic surface rheological parameters has been developed for the encapsulation of a microbubble contrast agent. A modified Rayleigh–Plesset equation with these parameters (surface tension γ and surface dilatational viscosity κs) was obtained. The rationale for the zero-thickness interface lies in the anisotropy and the inhomogeneity in the structure of the encapsulation. The parameters of the model were determined using in vitro attenuation data for various contrast agents (Albunex, Optison, Sonazoid, and Quantison). The linearized dynamics was used to match data from a controlled set of experiments performed at smaller pressure amplitudes (<0.1 MPa). The model was investigated for its ability to predict the nonlinear response at higher amplitudes (>1.0 MPa) for two test cases: Optison (Mallinckrodt, St. Louis, MO) with γ=0.9 N/m, κs=0.08 msP and Sonazoid (Amersham Health, Oslo, Norway) with γ=0.6 N/m, κs=0.01 msP. In contrast to existing models, the new model captured the distinct characteristics of subharmonic emissions—initiation and rapid growth—for these agents. The present model seems to be a reliable descriptor of contrast agent behaviors, and therefore, may be used to characterize different agents and to design next generation agents by tuning the model parameters. [Work supported by DOD.]