To manifest potentiality of an ac calorimetric method, successful application of a light-irradiation ac calorimetric method is introduced in the studies of phase transitions of quartz, ferroelectric BaTiO3, and two-dimensional antiferromagnetic K2NiF4-family. In quartz, the alpha-to-incommensurate and the incommensurate-to-beta phase transitions, which Lie in a narrow temperature range, were clearly separated by this method. In BaTiO3, the heat capacity at first-order transition was measured with high precision and, therefore, it was found that the detailed behavior at the first-order transition is well-expressed in terms of the free energy expansion. As a result, in a stress-clamped crystal in which the phase transition becomes second order, the discontinuity of the heat capacity at the phase transition was estimated to be 0.19 k(B). This result stimulated further theoretical investigation. From this study, it was revealed that BaTiO3 is classified as a typical displacement-type ferroelectric. In order to study crossover phenomenon from a two-dimensional Ising system to a two-dimensional Heisenberg system in the K2NiF4 family, the critical amplitude of the heat capacity was measured as a function of the strength of Ising-like anisotropy. Based upon the analysis for the results, the existence of an asymptotic transition temperature in a two-dimensional Heisenberg system is proposed.