Calorimetry and neutron scattering techniques are combined to study the structures and phase transitions occurring in various 2-D molecular solids formed at gas-solid and solid-liquid interfaces. The solid monolayers of methyl fluoride and methyl chloride adsorbed on the surface of graphite exhibit a phase transition, the former is of an order-order type and the latter of an order-disorder type. The 2-D molecular solids of various molecules including alkanes and carboxylic acids formed at an interface of the bulk liquids and graphite melt well above the bulk melting temperatures, whereas the 2-D solids formed on the surface of graphite melt well below the bulk melting temperatures. It is considered that the 2-D solids formed at gas-solid interface melt to form the 2-D liquids, whereas the 2-D solids formed at solid-liquid interface melt to form the bulk liquids.