A new method, single particle electrodynamic levitation SPEL, has been used to measure the water isotherm for microparticles of porous carbon. SPEL is an ultrasensitive gravimetric analyzer that uses electric fields to levitate single microparticles in a flowing gas stream. The quantity of adsorbed vapor is directly proportional to the electric field intensity required to suspend the particle at the center of the levitater. Water isotherms for nanogram microparticles of BPL and ASC carbon are measured at 25-degrees-C using SPEL and compared to bulk carbon isotherm data. It is shown that SPEL can successfully differentiate the ASC and BPL carbon water isotherms and replicate standard isotherm data with only a limited number of single particle measurements. Hysteresis effects are also measurable with SPEL.