Titanite is a common accessory phase in igneous rocks and is particularly abundant in the Half Dome Granodiorite of Yosemite National Park, California. Holding a large proportion of trace elements, such as rare earth elements, makes titanite an important phase because of the influence of temperature, pressure, oxygen fugacity, and liquid composition on trace-element partitioning. Equilibrium crystallization and fractional crystallization have been used to explain the distribution of elements in titanite. However, backscattered electron images of titanite reveal complex zoning patterns that correlate with rare earth element concentrations. Transects across the centers of titanite crystals further emphasize the fluctuating chemistry of titanite and its complex crystallization. The geochemical and textural evidence from titanite show that simple fractional crystallization models cannot explain the distribution of rare earth elements throughout the crystals.