In long base isolated buildings the superstructure may consist of several parts separated by narrow thermal expansion joints. In such cases, neighboring bearings which support adjacent buildings are connected together at their top so that they form a common isolation and prevent impact at the isolation level. The analysis of such buildings constitutes the problem of a multiple base-isolated structure with a common basemat and isolation system. This situation can not be analyzed with the existing algorithms which are capable of analyzing only a single base-isolated structure. The torsional character- istics of the combined system are different than those of individual buildings on individual isolation systems. Hence, the combined system of several buildings on a common isolation system needs to be analyzed in its entirety rather than analyzing each building with its isolation system separately. In this paper an analytical model and an algorithm to analyze multiple buildings on a common isolation system are presented. Verification of the accuracy of the algorithm by comparison with results obtained using a general purpose finite element program are presented. A multiple building base-isolated structure is analyzed and the results are used to demonstrate the importance of analyzing the combined system as against analyzing individual buildings.