The authors report in situ measurements of the reversible, electric field mediated assembly of colloidal gold microstructures and their associated impedance properties on surfaces between planar gold film microelectrodes. Video optical microscopy is used to monitor the assembly of wires and locally concentrated configurations having variable resistances and capacitances. A scaling analysis of dominant electrokinetic transport mechanisms at different electric field amplitudes and frequencies is consistent with the observed steady-state microstructures. Impedance spectra are fit to equivalent circuits with elements directly connected to physical characteristics of the microelectronic/fluidic device components and different particle microstructures. \ 2007 American Institute of Physics.