Crude oil asphaltenes represent a solubility fraction of petroleum that includes the most polar and most interface active compounds. Asphaltenes have been involved in solid deposition phenomena in oilfield operations and have produced impractical conditions for oil flow in pipelines and reservoir porous rock. In addition, asphaltenes contribute to the formation of both oil-in-water and water-in-oil emulsions. Therefore, the properties of asphaltene interfacial films are crucial to defining the stabilization or breaking of crude oil emulsions. In this work, the results are presented as a contribution to understanding the elasticity and phase behavior of asphaltene liquid films with changes in subphase pH and temperature. n-Pentane-insoluble asphaltenes (C5I) were extracted from a light crude oil and deposited at the air water interface from a dichloromethane spreading solution to form two-dimensional monolayers. The C5I films were evaluated using a Langmuir trough under compression at a constant rate. Surface pressure area and surface potential area isotherms were interpreted from a mathematic model of the surface pressure. The C5I monolayers displayed an extensive region containing liquid-expanded (LE) and liquid condensed (LC) phases and a well-identified transition region between these phases. The compressibility of LE films (0.01-0.02 m mN(-1)) was approximately 5-fold lower than the compressibility of LC films (0.05-0.07 m mN(-1)). The mixed film compressibility was between the LE and LC film compressibilities. At 10 degrees C, more compressed films were obtained at pH 4. At 30 degrees C, more compressed films were found at pH 8. Surface pressure (pi) was evaluated as a function of the subphase pH and temperature. At 20 degrees C, the lowest pi was between the pH 4 and 8 isotherms. The highest surface pressure was 52 mN m(-1) at 10 degrees C and pH 8, which denotes higher repulsive forces acting in the film. The elasticity of the asphaltene Langmuir films was affected by the ionization of acidic and basic groups on the asphaltene structure.