The Kmelia component model is an abstract formal component model based on services. It is dedicated to the specification and development of correct components. This work enriches the Kmelia language to allow the description of data, expressions and assertions when specifying components and services. The objective is to enable the use of assertions in \kmelia in order to support expressive service descriptions, to support client/supplier contracts with pre/post-conditions, and to enhance formal analysis of component-based system. Assertions are used to perfom analysis of services, component assemblies and service compositions. We illustrate the work with the verification of consistency properties involving data at component and assembly levels.