Triggered Message Sequence Charts (TMSCs) [16] are a scenario-based visual for-malism for capturing requirements of distributed systems. TMSCs enhance traditionalMSCs [3] with capabilities for expressing conditional and partial behavior and with amathematically precise notion of refinement, which may be used to check whether oneset of requirements correctly elaborates on another. This paper presents TRIM, a toolfor checking refinement between TMSCs. The main features of TRIM are: (i) a textuallanguage for TMSCs that includes the algebraic combinators of [16]; (ii) a routine forchecking refinements among TMSC specifications; and (iii) a capability for generatingdiagnostic information in the form of tests when one system fails to refine another.