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The stability region of the 1/1 cubic crystalline approximant to icosahedral quasicrystal in the Cd-Mg-Ce system was investigated in detail. The partial isothermal section of the ternary phase diagram at 673 K was determined in the range from 0 to 18 at% Ce, where it was shown that the approximant spans a wide compositional range of Cd 57−86 Mg 0−29 Ce 14–15. Mg addition was found to shift the stability region to lower temperatures. Besides, the approximant was found to be in equilibrium not with the Cd-Mg solid solution but with (Cd,Mg) 17 Ce 2 and (Cd,Mg) 11 Ce compounds as opposed to the previous work [J. Alloys Compd. 822 (2020) 153541]. Based on the diffraction data (both X-ray and electron), to our surprise, a new variant of 1/1 cubic approximant was identified within the compositional range. The prototype Cd 37 Ce 6 structure (space group Pn 3 ̅) which dominates near the Cd-rich end was found to be partly taken over by the Cd 25 Eu 4 -type structure (space group Fd 3 ̅) with a doubled unit cell in the higher-Mg part of the compositional range. The atomic structure with both Pn 3 ̅ and Fd 3 ̅ symmetries were determined by means of single-crystal X-ray diffraction measurement. Their refined atomic structures were compared and the preferential sites for Mg substitution are revealed. • Isothermal phase diagram of the Cd-Mg-Ce system at 673 K is developed. • Two variants of 1/1 cubic approximants are found using XRD and SAED. • Single crystals of the two variants are synthesized successfully. • Atomic structures of the two variants are revealed and compared using SCXRD method. • Very low chemical disorder, especially in the Pn 3 ̅ variant, is noticed. [ABSTRACT FROM AUTHOR]