The prevailing low-temperature phase (T less than or equal to 100degreesC) has been studied in binary mixtures of very long n-alkanes C122H246, C162H326, C194H390, C210H422, C246H494, C258H518 and C294H590. For alkane pairs with chain length ratio between 1.3 and 1.7 and molar ratios not too far from 1: 1, the lamellar structure is that of a superlattice. Its structure is determined by small-angle X-ray scattering (SAXS), backed by calorimetry and wide-angle X-ray diffraction. One full repeat unit of the superlattice contains three crystalline layers; the two outer ones contain both long and short molecules while the middle layer contains extended interdigitated tails of the long molecules protruding from the two outer layers. This triple layer phase forms through a reversible transition on cooling from the semi-crystalline form (SCF) described by Zeng and Ungar [Macromolecules, 34 (2001) 6945]. Both forms are solid solutions in which the disparate alkane chains are intimately mixed. SCF consists of alternating crystalline and amorphous layers, the latter consisting of surplus length of the longer alkane. The transition consists of the longer molecules from two adjacent crystal layers being pulled into the intervening space and crystallizing in the form of the middle layer; at the same time the opposite ends of the chains are being aligned flush with the outer surfaces.