This study has investigated aerosol transformation processes for four common sources of indoor ultrafine particles (UFP): a gas stove, an electric stove, a candle, and a hair dryer. For each of the four UFP sources, the temporal change in particle size distribution (3 nm to 100 nm) was measured during particle decay. An analytical model was developed based on the discrete general dynamic mass-balance equation considering coagulation, deposition, and ventilation. The results indicate that in addition to particle deposition and ventilation loss, coagulation is a dominant physical process for changes in indoor UFP size distributions. Especially at high UFP concentrations, coagulation was found to be a dominant process that transforms small particles to larger sizes in a short time.