Plasma composition measurements at ~2-min time resolution from 27 magnetopause crossings are used to determine if mass density &rgr;, modified by the anisotropic pressure term (1-&agr;), is constant across the magnetopause. In the process of testing this relation across the magnetopause using the low time resolution measurements, several new properties of the discontinuity and its layers are presented. Pressure and mass density in the magnetosheath and low-latitude boundary layer were found to be dominated by H+ except on one occasion (out of the 27 crossings) when O+ was the dominant contributor to the total mass density in the low-latitude boundary layer. Using these measurements, it is found that &rgr;(1-&agr;) is not constant for almost all magnetopause crossings even when there is independent evidence for magnetic reconnection and an open magnetopause. These results, when combined with previous observations at much higher time resolution, indicate that the reason why &rgr;(1-&agr;) is not constant across the magnetopause is that the H+ density decrease from the magnetosheath to the low-latitude boundary layer is not compensated for by either a change in the pressure anisotropy or a change in the total mass density and suggest that the magnetopause can never be described as a time-stationary one-dimensional rotational discontinuity. ¿ American Geophysical Union 1993