This paper uses experimentally determined solid-liquid phase relation studies within the MgCl2-H2O-C2H4(OH)(2) ternary system to identify the origin of scale which formed in the packing of the dehydration column of the Australian Magnesium (AM) process for producing anhydrous magnesium chloride. The scale formed during dehydration of a purified magnesium chloride (MgCl2) leach liquor plus recycled ethylene glycol (C2H4(OH)(2)) solution. Experiments at 333 K showed the following phase fields present: a liquid-only phase region (Liq(T)); three two-phase regions T + Liq(T), MgCl2 center dot 6H(2)O + Liq alpha, and MgCl2 center dot 3[C2H4(OH)(2)] + Liq beta; plus two three-phase regions MgCl2 center dot 6H(2)O + T + Liq alpha and MgCl2 center dot 3[C2H4(OH)(2)] + T + Liq beta. The compositions Liq alpha and Liq beta represent liquid compositions that extend a small distance into the ternary from the invariant points A and B located along the respective MgCl2-H2O and MgCl2-C2H4(OH)(2) binaries, while T represents a ternary phase of composition MgCl2 center dot 2H(2)O center dot 2[C2H4(OH)(2)]. An additional four three-phase regions were predicted based on the existence of known crystalline phases. These included: MgCl2 center dot 6H(2)O + T + MgCl2 center dot 4H(2)O, MgCl2 center dot 4H(2)O + T + MgCl2 center dot 2H(2)O, MgCl2 center dot 2H(2)O + T + MgCl2, and MgCl2 + T + MgCl2 center dot 3[C2H4(OH)(2)]. The ternary compound was identified as the solid that precipitated within the dehydration column. Additional testwork between (293 and 363) K indicated that the position of the solid-liquid boundary was relatively insensitive to temperature changes.