INVESTIGATIONS using a thermostated double-acting recording surface balance of the Wilhelmy-Dervichian type1,2 reveal an interesting series of phase changes in condensed monolayers of normal chain fatty acids of high molecular weight. As an example, we may take the surface pressure – area curves for n-docosanoic (behenic) acid3 spread on acid (0·01 N hydrochloric acid) substrate shown in Fig. 1. At the lowest temperature the solid film gives a smooth pressure – area curve and the area can be reduced down to about 18·6 sq. A. per molecule before a gradual collapse sets in. At this area the chains are evidently vertical and as closely packed as in the normal three-dimensional hydrocarbon crystal, in which the cross-section of the chains is about 18·4 sq. A. This close-packed solid phase may be denoted by CS. When the temperature is raised, another phase, L2, appears5, the stability range of which increases with temperature. At 8·2° a new phase, which we may call L'2, appears and is quite marked in the curve for 10° in Fig. 1. At 13·8° the high-pressure phase S appears and the curve for 14·8° shows a transition S r CS which is rapidly displaced towards higher pressures with raising temperature. Above 18°, CS has disappeared and S shows a sharp collapse at about 55 dynes pressure. The smallest area at which the phase S is stable is about 19·3 sq. A. It seems likely, therefore, that the molecules are vertical and perform strong torsional oscillations6 or complete rotations, as the area per chain corresponds to that found in hydrocarbon crystals near the melting point4. (A few degrees below the melting point some hydrocarbons show a gradual, others an abrupt, change from a state of lower symmetry into one of hexagonal symmetry, in which the vertical chains are rotating about the long axis.)