The effects of a bottom-mounted rectangular block on the sloshing characteristics of the fluid in rectangular tanks are investigated using the linear water wave theory. For the sloshing analysis the velocity potential is decomposed into those for the wall-induced waves and the reflected, transmitted and scattered waves by the block. The reflection and transmission coefficients related to the block are determined using the continuity conditions of mass flux and energy flux on the common vertical boundaries of the fluid regions and the boundary conditions on both sides of the block. Verification of the present analysis method is carried out by comparing the results with those by other methods. The analysis results indicate that the size and location of the block significantly influence the sloshing frequencies and mode shapes. In general, sloshing frequencies reduce as the block becomes tall and wide. The variations of the sloshing frequencies and mode shapes due to the block are found to be more sensitive in broad tanks than in tall tanks. The hydrodynamic pressure exerted on the tank wall and the block changes significantly as the block becomes large and moves toward the wall. Copyright (C) 1996 Elsevier Science Ltd.