The achievement of widely accepted standards for Open Systems Interconnection (OSI) is closely tied to the ability of producing unambiguous and implementation independent specifications of related protocols and services. LOTOS, the Language fOr Temporal Ordering Specification, is a Formal Description Technique (FDT) whose definition, though not completed, has already reached such a state as to allow trial specifications of rather sophisticated services and protocols. This paper explores the specification in LOTOS of some of the session services whose discussion is underway within various standardization bodies. Conciseness of specification is tried by adopting a few notational variants which are guessed to be useful at various OSI layers. The session services selected for this trial specification comprise the Basic Combined Subset (BCS) enriched with the Expedited Data service.