The level and type of molecular orientation present in mixed Langmuir-Blodgett thin films containing poly(3-alkylthiophene) and deuterated cadmium stearate molecules were examined by using transmission and reflection-absorption FTIR techniques. For films fabricated with polythiophenes comprised of butyl, hexyl, or octyl side chains, it was found that the cadmium stearate molecules prefer to assemble into well-ordered, phase-separated stacks with a molecular organization very similar to that observed in homogeneous LB films of this material. The polymer molecules, on the other hand, maintain a state of essentially random orientation over the entire compositional range of the mixed films. The most highly ordered domains of cadmium stearate molecules were found in films containing low polymer contents. For LB films fabricated with poly(3-octadecylthiophene), on the other hand, a higher level of molecular mixing with the cadmium stearate-d molecules, encouraged primarily by the longer alkyl side chain of the polymer, strongly inhibited the formation of well-ordered domains of cadmium stearate molecules, particularly at high polymer loadings. In this case, the hydrocarbon groups of the polymer molecules were found to be more oriented to the surface normal than the hydrocarbon tails of the cadmium stearate-d molecules, and the level of orientation actually increased with increasing polymer content. In certain cases, dichroic ratios were used to estimate relevant orientation angles. © 1990 American Chemical Society.