The integration of instruments with the grid promotes scientific and business collaboration by allowing shared access to rare and expensive instrumentation, regardless of its physical location. Instruments on the grid are virtualized resources similar to storage and computing devices, and can be part of composite services or workflows concurrently invoked by multiple users in such service-oriented environments. The requirement for real-time, interactive control of instruments poses a need for exclusive or limited access by concurrent clients. Unfortunately, instrument modeling and standardization of instrument reservation and management are still in a very early stage. According to a number of use case requirements, which we review in this work, we propose a standards-compliant model for the representation of instruments on the grid, and we describe a set of primitives for their reservation. The definition of these constructs can be used for the normative description of such reservation capabilities and the future specification of a standardized protocol.