Using data from the Active Magnetospheric Particle Tracer Explorers/Charge Composition Explorer spacecraft, we make a detailed comparison between the observed polarization properties of electromagnetic ion cyclotron (EMIC) waves and those predicted by theory. The polarization can be described by three parameters: the ellipticity &egr;, the ratio of parallel (to the background magnetic field B0) magnetic fluctuations ΔBz to the major axis component of the elliptical perturbation in the perpendicular plane ΔBmajor, and the phase angle between ΔBz and ΔBmajor. On the basis of the plasma parameters observed during EMIC events, we have calculated the linear properties of the theoretical modes and compared these to the observations. The result is that two and in some cases, three of the observed polarization properties are inconsistent with the assumption that the waves result from a single linear mode. We use a simple model with two constituent waves in various azimuthal orientations (around B0) and temporal phase relations and show that the distribution of observed polarization properties can be understood as resulting from the superposition of more than one mode. When there is superposition, the instantaneous polarization characteristics of the fluctuations do not reliably reflect the constituent wave properties and the minimum variance direction cannot be associated with a wave vector direction. Nonetheless, we have shown that the constituent wave properties can be inferred from the distribution of observed properties. For superposition of two waves with only slightly dissimilar characteristics, the constituent wave E is approximately the median observed E, E¿, and the constituent &thgr;kB (angle between the wave vector k and B0) is approximately given by tan &thgr;kB=ΔBz/ΔBmajor/E¿, with the overbar on ΔBz/ΔBmajor again indicating a median value. ¿ American Geophysical Union 1996