The seismic design and assessment of structures built in refinery installations present a number of challenges to the design engineers considering that the consequences of a seismic event would be severe compared to conventional civil structures. The structures built in these plants can be generally grouped into building structures and non-building structures. A primary structure in steel or reinforced concrete supporting non-building structures such as reactors, equipments etc. known as “coupled structure-equipment” represents a unique class from the perspective of seismic performance assessment. The seismic response of coupled structure-equipment is essentially controlled by the interactions between the supporting structure and supported non-building structures. The paper presents the seismic performance assessment study carried out on a typical gas phase reactor structure consisting of reinforced concrete primary structure supporting on its top a vertical reactor vessel. The structure is adapted from a refinery complex built in a region of moderate seismicity. A coupled model representing the structure and the vertical vessel is used for the study. The seismic response is evaluated using both the nonlinear static procedure and incremental dynamic analysis. The performance evaluation is done using FEMA 356 guidelines. The study shows that the reactor structure satisfies the intended performance objectives under earthquakes with the corresponding hazard levels.