This study examines the dynamic and thermodynamic characteristics associated with the evolution of the SCSSM of 1998 using the data from the South China Sea Monsoon Experiment (SCSMEX). The objective is to identify the physical processes that determined the active and inactive periods of the 1998 SCSSM.The intraseasonal variations in the activity of the 1998 SCSSM are found to be mainly controlled by the 30-60-day oscillation, but modified by the 10-20-day mode. Definition of the active and inactive period is thus based on the OLR anomaly and its 30-60-day filtered component, giving two active (25-30 May, 23 June-5 July) and two inactive periods (6-18 June, 13-26 July).An examination of the circulation features suggests that the two modes operated very differently. After the onset of the SCSSM, a northward propagation of the 30-60-day mode into the SCS dictates the active and inactive phases of the SCSSM. During an active (inactive) phase, positive (negative) relative vorticity and convergence (divergence) associated with a low-level trough (ridge) propagated into the SCS, initiating (suppressing) the convection. The divergence field at the upper level was in phase with the low-level relative vorticity. However, the direction of propagation associated with the 10-20-day mode was different. A westward propagation of this mode into the SCS caused changes in the convection over the region during the first half of the 30-60-day mode. A second oscillation apparently propagated from the midlatitudes into the SCS to initiate or suppress convection. The dynamic conditions in terms of the vorticity and divergence are similar to those for the 30-60-day mode. The atmosphere over the SCS was also found to be generally favourable for the development of deep convection after the onset of the SCSSM.