At a stationary temperature gradient, the grain boundaries comprising residual melt can migrate. Their migration is governed by liquid diffusion and, in multiphase materials, depends highly on the phases present. Therefore, this phenomenon can be used for phase identification if data on metastable extensions of the corresponding phase diagram are available. Thus, we re-optimised the thermodynamic description of Tris(hydroxymethyl)aminomethane-Neopentylglycol (TRIS–NPG), a transparent peritectic alloy often used as a model alloy for metallic solidification, and used the predicted metastable liquidus and solidus curves to evaluate the grain boundary migration observations. As we found temperature gradient zone melting (TGZM) at low temperatures, the presence of the peritectic phase could be excluded even though a near-peritectic alloy had been processed. The liquid diffusivity, as a function of the position/temperature, was estimated from the TGZM velocity measurements. The data suggest that the diffusion coefficient deep in the mush is one order of magnitude smaller than that close to the liquidus temperature. This may be typical for non-dilute alloys, where the concentration of the intergranular liquid changes considerably.