Insoluble monolayers of long-chain acids at the water surface are unstable above their equilibrium spreading pressure (ESP). They exhibit pressure relaxation at a constant area or area relaxation at a constant pressure. The main cause of these relaxations for insoluble monolayers is the formation of three-dimensional structures at higher surface pressures by compression. The shape of pi-A isotherms of a series of long-chain acids measured for a constant observation time at 10-degrees-C changed systematically with changing chain length. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) observation of one-layer samples deposited onto cover glass revealed that there are two kinds of three-dimensional structure. The nucleation and growth mechanisms of these three-dimensional structures are proposed from the TEM and AFM images.