We show how a formal framework for <em>observation</em> in computer systems can be used for the specification of agent's observable behaviour, and provide for an architecture-independent approach alternative to the one currently used for FIPA ACL. Since the agent architecture induced by our model is more abstract than the one assumed by FIPA, our specification tool is likely to be applicable to a wider set of agents' architecture, thus better supporting FIPA standardisation aims. Some application examples are shown, describing how the current semantics of FIPA ACL could be adapted and specified within our framework.