Nitrogen adsorption isotherms on organized mesoporous aluminas prepared by several different synthesis procedures were analyzed by means of comparative plot method using Aluminiumoxid C (Degussa) and alpha-alumina as reference adsorbents. To secure the high-resolution ability of this method, all the adsorption measurements were carefully performed in a relative pressure range from 10(-6) to 0.99. Although some samples of organized mesoporous alumina were treated at temperatures up to 1000 degrees C, only the Aluminiumoxid C has proved to be suitable as a reference adsorbent. The comparative analysis of isotherms on activated aluminas has shown that this method allows the determination of very small amounts of microporosity. The standard nitrogen adsorption data for Aluminiumoxid C and alpha-alumina are presented in a tabulated form, which consists of 91 points for each adsorbent.