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In this paper we report attempts to directly detect the behaviors of hepatitis B virus surface antigen (HBsAg) binding procedures and interactions between HBsAg and hepatitis B virus surface antibody (HBsAb) on the liquid–solid interfaces. In the work, an optical transmittance interferometry based transducer was used to detect the film thickness change in real time. Several hundred nanometer polystyrene films were deposited on the glass slides to construct polymer matrix and HBsAg was immobilized on their surfaces through physisorphtion. At the incubation end, film thickness increase 5.7, 4.3, 3.1, 2.4 and 1.2 nm at the HBsAg concentrations of 5, 4, 3, 2 and 1 mg ml−1, respectively. On the 5 mg ml−1 HBsAg coated film surfaces, HBsAg–HBsAb interactions resulted in 5.3, 3.4, 2.7, 1.8 and 0.6 nm film thickness increase at the concentrations of 15, 20, 36, 50, and 100 μg ml−1, respectively. The mass transport effects and reaction effects were obtained using the biosensor. Nonlinear regression analysis indicated that the association constant between HBsAg and HBsAb is 9.5×104 M−1s−1 and the dissociation rate kd is 1.21×10−3 s−1, respectively.