Determination of kinetic and thermodynamic protein binding constants using interferometry from a porous Si Fabry-Perot layer is presented. A protein A capture probe is adsorbed within the pores of an oxidized porous Si matrix, and binding of immunoglobulin G (IgG) antibodies derived from different species is investigated. The relative protein A/IgG binding affinity is human > rabbit > goat, in agreement with literature values. The equilibrium binding constant (Ka) for human IgG binding to surface-immobilized protein A is determined to be (3.0 +/- 0.5) x 107 M-1 using an equilibrium Langmuir model. Kinetic rate constants are calculated to be kd = (2.1 +/- 0.2) x 10-4 s-1 and ka = (1.2 +/- 0.4) x 104 M-1 s-1 using nonlinear least-squares analysis, yielding an equilibrium binding constant of Ka = (5.5 +/- 1.5) x 107 M-1. Both steady-state and time-dependent measurements yield equilibrium binding constants that are consistent with literature values. Kinetic rate constants determined through nonlinear least-squares analysis are also in agreement with protein A/IgG binding on a surface. Dosing with a high concentration of analyte leads to deviations from ideal binding behavior, interpreted in terms of restricted analyte diffusion within the porous SiO2 matrix. It is shown that the diffusion limitations can be minimized if the kinetic measurements are performed at low analyte concentrations or under conditions in which the protein A capture probe is not saturated with analyte. Potential limitations of the use of porous SiO2 interferometers for quantitative determination of protein binding constants are discussed.