We study the large-time behavior of weak solutions for the equations of isothermal gas dynamics: uf + (k2/v)= = vt -ux = 0, when the initial data have bounded total variation. The weak stability theorcm of T. P. Liu says that the solution converges to the solution to the Pdemann problem whose initial data are composed of (u0(+oz), v0(=t=~)). The airo of this note is to give the rate of convergence, which is to study the large-time behavior of strong shock waves interacting with weak waves. If two strong shock waves emcrge, the speed and the strength of these shock waves approach thosc of the solution to the Riemann problem at the tate t-~(~ > 0). If a single shock wave emerges, the speed and the strcngth of this shock wave approach at the rate of t -3/2 and the total variation of the solution outside the strong shock approaches zero at tiro rate t -~/'2.