The objective of this study was to characterize the changes in the rankings of the top-n results of major search engines over time and to compare the rankings between these engines. We considered only the top-ten results, since users usually inspect only the first page returned by the search engine, which normally contains ten results. In particular, we compare rankings of the top ten results of the search engines Google and AlltheWeb on identical queries over a period of three weeks. The experiment was repeated twice, in October 2003 and in January 2004 in order to assess changes to the top ten results of some of the queries during a three months period. Results show that the rankings of AlltheWeb were highly stable over each period, while the rankings of Google underwent constant yet minor changes, with occasional major ones. Changes over time can be explained by the dynamic nature of the Web or by fluctuations in the search engines’ indexes (especially when frequent switches in the rankings are observed). The top ten results of the two search engines have surprisingly low overlap. With such small overlap (occasionally only a single URL) the task of comparing the rankings of the two engines becomes extremely challenging, and additional measures are needed to assess rankings in such situations.