We introduce a new techniques-scanning activity gravimetric analysis (SAGA)-for investigating phase transitions in semicrystalline polymers. Isothermal growth and dissolution of polymer crystallites within picogram to milligram samples are manifested by mass changes in response to changes in the activity of sorbed solvent vapor. Single charged particles are levitated and weighed in an electrostatic field, providing access to highly supersaturated states. Phase transitions are inferred from simultaneous sorption and light scattering measurements. Analogous to differential scanning calorimetry, scanning solvent activity up and down exposes broad transitions between the semicrystalline solid state and the dissolved state, which are influenced by sample history. We demonstrate repeated dissolution and crystallization of an 11 ng sample of poly( ethylene oxide) by controlling the activity of sorbed water vapor and observe self-nucleation of crystallites from partially and fully solvated states.