Equations for parameter identification based on sinusoidal response information at two test frequencies are developed for identifying three unknown parameters including the gain constant, and either a pair of poles, or a pole-zero combination of the open-loop transfer function of a system. The technique of phase equivalent reduction is applied at the two test frequencies to simplify the derivation of the identification equations. The method is extended for the general problem of identifyingnunknown parameters. The generalized procedure is illustrated for identifying three and four unknowns. Limitations of the method, in applying it to higher order systems, are pointed out.