Web services composition is an emerging paradigm for enabling application integration within and across organizational boundaries. In this context, we propose an approach based on Symbolic Observation Graphs (SOG) allowing to decide whether two (or more) web services can cooperate safely. The compatibility between two web services is defined by the well known soundness property on open workflow nets. This property guarantees the absence of anomalies (e.g. deadlock) that can appear after composition. We propose to abstract the concrete behavior of a web service using a SOG and show how composition of web services as well as the compatibility check can be achieved through the composition of their abstractions (i.e. SOGs). This approach allows to respect the privacy of the services since SOGs are based on collaborative activities only and hide the internal structure and behavior of the corresponding service.