In this article we present work done developing an interactive comparison tool for large-scale catalogs using the general purpose open source digital library toolkit, Greenstone. The two catalogs selected to demonstrate the approach were the Bibliographic Records from New Zealand's National Bibliography and the HathiTrust. With Greenstone's triple-store extension activated, the two collections were ingested to form two Greenstone collections. Next, an interactive visualization tool was developed within the digital library's presentation layer to allow users to explore the two collections, comparing fields from the two collections and producing a variety of visualizations. The required interactivity was accomplished using AJAX calls to the Greenstone triple-store, further supported by the use of Javascript libraries for the presentation of the retrieved data in both visual and spreadsheet forms.