A study is reported on Langmuir monolayers and deposited Langmuir-Blodgett (LB) films obtained by mixing, at different molar ratios, stearic acid and poly(3-n-butylthiophene) (P3BT). Poly(3-n-butylthiophene) was synthesized chemically, the resulting polymer possessing an average molecular weight of 250 000 g mol-1 (hereafter high weight HW P3BT) or 15 300 g mol (hereafter low weight LW P3BT) when FeCl3 was not previously dried. From surface pressure-area curves, the average area per molecule (of stearic acid) was found to increase with the molecular ratio P3BT:SA. The collapse pressure for the LW P3BT:SA monolayers is above 70 m Nm-1 and is practically independent of the P3BT:SA ratio. The most important feature in these pi-A curves is a minimum following collapse. For the HW P3BT:SA monolayers, however, the collapse pressure decreases with increasing P3BT:SA ratio, and is significantly lower than that of the LW P3BT:SA monolayers. Collapse is characterized by a change in gradient of the rise in surface pressure, and no minimum is observed. The surface potential of the mixed films is higher than the value for pure stearic acid, being generally above 400 mV. High transfer ratios were obtained for LB deposition using the vertical dipping method. Although the instantaneous transfer ratio shows a number of oscillations, the accumulated transfer ratio increases linearly with time.