The processes of surface modification on gold-coated piezoelectric quartz crystal sensor have been in situ investigated by using the impedance analysis technique. The changes of equivalent circuit parameters were used to interpret the different behaviors of successively modifying protein, cross-linker, and DNA onto a gold electrode surface. It was found that the frequency changes due to protein adsorption might be described as a sum of two exponential functions, compared with a first-order kinetic function exhibited in subsequent binding of cross-linker to protein. The kinetic parameters were fitted to these two cases. It was also shown that the DNA molecules binding to cross-linker gave rise to the changes of the density-viscosity and the dielectric constant, both being linearly related to DNA concentration in the liquid when below 6.0 mu g m(-1).