The effect of poly(ethylenimine) (PEI) dissolved in water on the surface pressure-area (pi-A) isotherm of stearic acid (STA) at the air/water interface was investigated. When the concentration of PEI was quite low, for example, 2.2 x 10(-4) g/L (5.1 x 10(-6) M in monomeric unit concentration), the isotherm of the STA monolayer was little affected by PEI. On a concentrated PEI solution, for example, at 2.2 x 10(-2) g/L, the isotherm of the STA monolayer exhibited a noticeable variation as a function of subphase pH; particularly in basic conditions. The isotherm of STA could be obtained very reproducibly even at pH 10.4. At basic pHs, PEI induced the occurrence of a wide plateau region in an isotherm that could be attributed to the coexistence of liquid-condensed (LC) and liquid-expanded (LE) phases of STA on a water subphase. In addition, the collapse pressure of the STA monolayer was raised to 68 mN/m, indicating that the stability of the monolayer could be increased dramatically by virtue of an acid-base-type interaction between the amine group of PEI and the carboxyl group of STA. In situ reflection-absorption infrared (RAIR) spectroscopy revealed that at acidic pi-Is STA molecules should form two-dimensional crystalline domains even at high "per molecular" area. In contrast, at basic pHs RAIR spectral data dictated that STA molecules were strongly disordered at high per molecular area, and a near-crystalline structure seemed to form only when the per molecular area was lowered to a value corresponding to a pure LC phase in the pi-A isotherm of STA on the PEI-containing subphase.