The Grid is inherently unreliable due to its geographical dispersion, heterogeneity and the involvement of multiple administrative domains. The most general case of failures are so-called Byzantine failures where no assumptions about the behavior of faulty components can be made. In this paper a novel system is described that allows to diagnose and tolerate byzantine faults based on service replication. We suggest, briefly describe and compare two fail-stop and two byzantine fault tolerance algorithms. Given that many scientific larger-scale Grid applications have complex outputs the comparison of replica results as needed to implement byzantine fault tolerance becomes a non-trivial task. Therefore we include an automation mechanism based on a generic description language and code generation for this particualar problem. Our approach has been implemented as extension to the Otho Toolkit, a system that synthesizes tailor-made wrapper services for a given application, Grid environment and resource. An analysis of performance and overheads for three real-world applications completes our work.