An interfacial stress rheometer has been constructed to study the rheology of Langmuir films subjected to time-dependent flows. A magnetized rod resides at the air−water interface and is set into oscillation by applying a sinusoidal magnetic field gradient. Analysis of the amplitude and phase of the resulting rod motion relative to the applied force allows the determination of the dynamic surface modulus, Gs*(ω), and measurement of the relative elastic and viscous contributions of the monolayer. Measurements at 22 °C were conducted on eicosanol (C20) and mixtures of a rigid-rod polymer, phthalocyaninatopolysiloxane (PcPS), dispersed in eicosanol. The surface pressure dependence of the rheology for eicosanol reveals the presence of a maximum in the loss modulus, Gs‘ ‘(ω), within the L2‘ phase at Π = 6 mN/m. In the LSI phase at pressures above 15 mN/m, the monolayer is Newtonian and has a surface viscosity of 0.03 mN·s/m. The mixtures of PcPS with eicosanol are known to have two-dimensional nematic behavior...