High-temperature stable lamellar structures composed of 5-(octadecyloxy)isophthalic acid (C(18)ISA) and 5-(dodecyloxy)isophthalic acid (C(12)ISA), specifically deuterated in the alkyl chain (C(18)ISA-d(37)), aromatic head group (C(12)ISA-d(3)), or acid moieties (C(18)ISA-COOD), were studied with solid-state H-2-NMR spectroscopy. Combined with results from earlier work, it is concluded that apolar interactions between the long alkyl chains in C(18)ISA stabilize the solid on heating until 433 K and the liquid-crystalline phase on cooling, even when contributions from the hydrogen bonds are weak. The apolar interactions between the shorter alkyl chains in C(12)ISA are weaker, and the liquid-crystalline phase present on cooling is destabilized. The C(n)ISA systems thus provide promising high-temperature model membrane structures, due to the possibility of balancing the apolar interactions (alkyl chain length) and polar interactions (H-bonding of the head group).