We investigated the search behaviour of the library searcher using the log data from The European Library (TEL). We were especially interested in how this behaviour compares to the search behaviour of ad-hoc searchers, represented by log data in the MSN search engine. At first sight, the two data sets mainly differ in the topics of the queries entered and their multi-lingual vs. mono-lingual content. When studying user behaviour, session information is very important: how does the user navigate through the engine’s interface? We visualized the TEL users’ interactions with the system by creating a transition network for the users’ intra-session actions. In general, we think that research into user behaviour on the basis of search engine logs can be very informative for the evaluation of search engine interfaces.