Measurements of the mass and surface charge of microparticles are employed in the characterization of many types of colloidal dispersions. The suspended microchannel resonator (SMR) is capable of measuring individual particle masses with femtogram resolution. The high sensitivity of the SMR resonance frequency to changes in particle position in the SMR channel is employed to determine the electrophoretic mobility of discrete particles in an applied electric field. When an oscillating electric field is applied to the suspended microchannel, the transient resonance frequency shift corresponding to a particle transit can be analyzed to extract both the buoyant mass and electrophoretic mobility of each particle. These parameters, together with the mean particle density, can be used to compute the size, absolute mass, and surface charge of discrete particles.