When estimating rate constants using the BIAcore surface plasmon resonance (SPR) biosensor, one must have an accurate mathematical model to interpret sensogram data. Several models of differing complexity are discussed, including the effective rate constant (ERC) approach. This model can be shown formally to be good within O(Da) in the limit of small Damköhler number Da, which is the ratio of the reaction rate to the rate of transport to the surface. Numerical results are presented that show that except for very slow reactions, parameter estimates from the ERC model are very close to those estimated using a more complicated model. The BIAcore measures the behavior of an evanescent wave whose signal strength decays as it penetrates into the device. It is shown that this decay does not appreciably affect the sensogram readout at low Da, but at moderate Da can lead to situations where two vastly different rate constants can produce the same short-time sensogram data.