The Al 3 Cu 4 alloy, with an e=a ratio of 1.86 being close to ternary Al-Cu-TM (transition metal) quasicrystals, has been chosen for the search of Al-Cu approximants. Phase structures and compositions were studied using TEM, X-ray diffraction and EPMA techniques. Two new phases were found: face-centered orthorhombic oF-Al 43.2 Cu 56.8 a 0:816 6 ; b 1:414 9 ; c 0:999 5 nm and body-centered orthorhombic oI-Al 41.3 Cu 58.7 (oI, a 0:408 3 ; b 0:707 4 ; c 0:999 5 nm: Their e=a ratios are the same as that of the Al-Cu-Fe icosahedral quasicrystal. Both are B2 superstructures and their unit cell components can be expressed approximately as oF-Al 36 Cu 48 A 12 and oI-Al 8 Cu 12 A 4 (A: vacancies). They both exist in twinning variants of the types 120Њ=001 and 180Њ=310: Such twinning modes indicate that these orthorhombic phases are the decomposition products of a high-temperature parent phase e2-Al 2 Cu 3 , the atomic structure of which shows pentagonal atomic arrangements. Further analysis on the twinning modes of oF and oI leads to the recognition of the chemical-twinning mode of the basic B2 structure as 180Њ=͗111͘ B2 : This kind of chemical twinning mode is responsible for the pentagonal atomic configuration in the Al-Cu approximants as well as for the pseudo-5-fold B2 twinning.