Abstract: An absolute method for single aerosol particle mass and charge measurement has been developed. The method involves suspension of a charged particle in an electrodynamic chamber called a quadrupole. From the voltage required to balance the particle against gravity, the particle charge-to-mass ratio can be measured. By irradiating the particle with ultraviolet light, a single electron can be removed and a new balancing voltage determined. From the balancing voltages measured before and after this so-called process of "electron-stepping," the particle mass and charge are determined absolutely without requiring knowledge of the particle shape, density, or any adjustable parameters. Excellent agreement is obtained by comparing the measured mass of calibration particles of known size.