In this paper, we describe a capture-recapture experiment conducted on Google's and MSN's cached directories. The anticipated outcome of this work was to monitor evolution rates in these web search services as well as measure their ability to index and maintain fresh and up-to-date results in their cached directories. Categories and Subject Descriptors H.3.5 [Information Storage and Retrieval]: Online Information Services ? web-based services, H.5.4 [Information Interfaces and Presentation]: Hypertext/Hypermedia General Terms: Algorithms, Measurement, Performance