Recently a new interesting technique, modulated differential scanning calorimetry (MDSC), was published. This technique is based on Fourier transformation, and its sound applicability depends on linearity in thermal processes, because the Fourier transformation is based on linearity or the superposition principle. If a process under observation is a non-linear process, the physical meaning of the Fourier transformation should be closely examined, The DSC output signals of some processes, such as melting and reaction, have been revealed to be non-linear. Therefore, differentiation of linearity from non-linearity in DSC thermal responses is very important for modulated DSC, and they are discussed in this paper. In this discussion, methods to analyze oscillating output signals are also proposed.