Much effort has been expended recently to investigate photocatalytic mechanisms by in situ spectroscopy. We introduced an alternative method to study photocatalysis thermodynamically by tracing thermodynamics and kinetics in real-time using a light-emitting diode photocalorimeter that includes a commercial microcalorimeter and fiber optics. We were almost able to adjust its baseline to zero by tuning the incident light intensity. After calibration, we used the photocalorimeter to trace heat changes and kinetics in the photocatalysis of methylene blue over a visible-light-driven Ag@AgCl photocatalyst. Three main heat changes occurred: (i) photoresponse of methylene blue molecules and photocatalyst, (ii) balance between endothermic photoresponse and pollutant photodegradation, and (iii) stable exothermic stage in pollutant photodegradation. The photocalorimeter underwent an initial endothermic reaction and a subsequent exothermic stage, and finally maintained a stable exothermic rate of -(0.1543 +/- 0.0446) mJ s(-1) with a pseudo-zero-order process. The heat effects of stages (i), (ii), and (iii) are 73.8, 143.9, and -1939.2 mJ, respectively. The photocatalytic mechanism was discussed in detail. Results from this study are critical to the development of promising applications in in situ photochemistry, physics, and biomedicine. (C) 2015 Elsevier B.V. All rights reserved.