The concept of an effective thermal capacitance for a thermal system has been investigated analytically. This parameter is not a constant characteristic of a system but varies for both steady-state and transient conditions. Representative results illustrating these chsnges have been obtained for (i) a closed system of a wall and the adjacent air layer and (ii) an open system of a solar-energy collector. The steady-state effective thermal capacitance for a solar-energy collector was found to be weakly dependent on the applied conditions. Thus it can be assumed as constant for most engineering purposes. However, under transient conditions it undergoes significant variations. The validity of various methods previously used to determine the effective thermal capacitance is discussed.