Abstract Stoichiometry of undoped LEC GaAs has been studied by improved coulometric analysis. The solidus phase boundary shape of GaAs has been predicted, based on experimental results. The congruent point has been found to be on the slightly arsenic-rich side. The solidus boundary shape, extending on the gallium-rich side, has been suggested to be flat, while the other boundary, extending to the arsenic-rich side, has been suggested to be steep. The influence of temperature fluctuations, in the vicinity of the growing crystal-melt interface, on compositional variation along the crystals has been clarified. The magnetic field applied LEC (MLEC) GaAs crystals showed a clear relationship with a predicted solidus phase boundary shape, whereas conventional LEC crystals showed an ambiguous correlation.