The growing synergy between Web Services and Grid-based technologies [7] will potentially enable profound, dynamic interactions between scientific applications dispersed in geographic, institutional, and conceptual space. Such deep interoperability requires the simplicity, robustness, and extensibility for which SOAP [4, 3] was conceived, thus making it a natural lingua franca. Concomitant with these advantages, however, is a degree of inefficiency that may limit the applicability of SOAP to some situations. In this paper, we investigate the limitations of SOAP for high-performance scientific computing. We analyze the processing of SOAP messages, and identify the issues of each stage. We present a high-performance SOAP implementation and a schema-specific parser based on the results of our investigation. After our SOAP optimizations are implemented, the most significant bottleneck is ASCII/double conversion. Instead of handling this using extensions to SOAP, we recommend a multiprotocol approach that uses SOAP to negotiate faster binary protocols between messaging participants.