This paper proposes a polarimetric method of determining the optimum conditions for photon excitation of surface plasmons (SPs). The method makes it possible to improve the accuracy with which the effective optical parameters of conductive samples are determined by an order of magnitude by using the phenomenon of zero reflection, which occurs for optimum excitation of SPs. The method is checked by means of an LEF-3M ellipsometer, in which a rotatable phase plate was used as an adjustable compensator. Measurements were made for an opaque copper mirror using a PCSA ellipsometric system with fixed polarizer at a probe-radiation wavelength of 0.6328 μm. © 2001 The Optical Society of America.