Pen-based interfaces offer exciting opportunities in ubiquitous computing by enabling new hardware form factors and socially acceptable computing tasks. However, prevailing ink representations are not compatible across devices or even within vendors, compromising interoperability and hence true ubiquity. We propose a lossless multimodal multi-format XML ink framework called RiverInk that overcomes many of the interoperability issues faced in using platform-specific standards. This paper motivates the interoperability problems created by ubiquity, and then describes the design of RiverInk?s format, APIs, and ink controls. RiverInk?s framework encompasses interoperable support for both archival and streaming ink-based applications, and a range of alternatives for platforms that do not currently support ink. Three RiverInk applications were developed within the ActiveCampus ubiquitous computing environment to demonstrate both the ease of adding interoperability to an ink-based application and the degree of interoperability provided. The adaptability of the framework to new platforms is discussed.