The melt-spun Mg39·5Ga16·4A14·1Zn40·0ribbon consists entirely of micrograins of an icosahedral phase (I-phase). The calorimetric and X-ray diffraction studies have shown that the I-phase undergoes an exothermic transformation to an approximant crystalline phase at 630 K on heating at a rate of 20 K min−1. Detailed structural analysis of the crystalline phase has revealed that the phase corresponds to a 3/2–2/1–2/1 rational approximant of I-phase, which has the orthorhombic C-type Bravais lattive with large cell parameters, a = 36·93 ± 0·06 Å, b = 22·83 ± 0·04 Å andc= 22·96 ± 0·04 Å.