Monitoring web services allows to analyze and verify some desired properties that services should exhibit. Such properties can be revealed by analyzing the execution of the services. Specifying monitoring expressions and extracting relevant information to perform monitoring is however not an easy task when the processes are specified by means of BPEL. In this paper we design a monitoring approach that makes use of business protocols as an abstraction of business processes specified by means of BPEL. High level queries are expressed against this abstraction and then translated into SQL queries that are evaluated against a database that stores the excustion traces of the services.