In-situ monitoring of refractive index changes during a liquid-solid phase transition is achieved by measurement of the transmission spectrum from a single tilted fiber Bragg grating immersed in water. Differential wavelength shifts of multiple mode resonances are used to eliminate cross-talk from temperature, throughout the phase transition, and from strains occurring after solidification. The measured sudden shift of refractive index at the phase transition is shown to be consistent with the expected difference from water to ice, in spite of the observed onset of compressive strain on the fiber by the frozen water. Beyond the obvious application to research on the dynamics of liquid-solid phase transitions, this work demonstrates the multiparameter measurement capabilities of multiresonant gratings.