Interactions between heat and mass diffusion determine growth mechanisms during ice crystallization. The effects of heat and mass transfer on ice growth in pure water and magnesium sulfate solution were investigated by studying the evolution of the gradient of the refractive index using color Schlieren deflectometry. For pure water, the gradient of the refractive index of water was used to calculate the temperature and therefore the local supersaturation. Its effect on the ice crystal growth rate and morphology was studied. It was found that, for local supersaturations greater than 2.8, the morphology was dendritic ice, with a growth rate 2 orders of magnitude higher than that for layered growth. During dendritic growth, 3–16% of the heat of crystallization diffused to the liquid side, which is counter to current understanding. At the transition (between the time of partial melting of the dendritic ice and the beginning of the layered ice growth), a higher supersaturation than that responsible for layered growth was observed. For ice growth from an aqueous salt solution, a mass and thermal diffusion boundary layer in front of the growing ice was created by diffusion of the solutes from the ice and by the release of heat of crystallization.