This paper is concerned with the impact of languagelevel communication mechanisms on the overall performance of distributed systems. Analytic models are developed of several existing mechanisms, including RPC and several enhancements. Using this modelling, a novel construct, called responsibilities, is synthesized. Analysis of the construct, guiding refinement of the initial design, demonstrates how the mechanism provides an improved impact on system performance compared to previous technology. Keywords distributed systems, performance modelling, programming language constructs 1 Introduction The time performance of the majority of distributed systems is constrained by the cost of communication. Computational elements of such systems are often idle, waiting for requests; or blocked, waiting for results. This paper explores language-level communication mechanisms which minimize these delays and their effects on overall system performance. A new construct is synthesized which merg...