Modulated differential scanning calorimetry (MDSC) has recently seen a large growth due to the availability of commercial instrumentation. In this paper it will be shown that MDSC based on heat flow presents a linear system as long as the conditions of measurement are carefully controlled, It will further be proven that for typical MDSC, a steady state can be maintained for heat capacity increases many times the typical increase of heat capacity with temperature, This permits not only measurement of heat capacity with continuous increase of temperature, but also the testing of glass transition regions and shallow endotherms as seen at the beginning of melting for nonreversible components. Finally, it will be shown that it may be possible to use a complex heat capacity for the description of the glass transition, but that the imaginary part of the heat capacity seems not to refer to any meaningful quantity.