We have studied the thermal melting of two disk-shaped molecules at the air-water interface. Brewster angle microscopy images of both compounds indicated first order phase transitions from a low-temperature solid phase to a high-temperature fluid phase. The high-temperature phase could be either isotropic or nematic. Grazing-incidence diffraction measurements provided corroborating evidence of reversible melting, in the form of Bragg peaks that vanished at high temperature and reappeared upon cooling. Furthermore, we observed that the diffraction peaks obtained upon recooling were substantially sharper than those obtained initially, and the domain sizes seen in Brewster angle microscopy were substantially larger, indicating an enhancement of crystalline domain size by thermal annealing.