Abstract A high sensitivity heat conduction microcalorimeter was manufactured and calibrated to be used in the determination of the immersion heats of activated carbon in aqueous solutions of phenol. A calibration constant between 13.15 ± 0.43 W V−1 and 15.24 ± 0.28 W V−1 was obtained for the manufactured microcalorimeter. Additionally, the accuracy of the microcalorimeter was verified by means of the determination of the neutralization heat of the HCl–NaOH system. A value of −56.64 ± 0.01 kJ mol−1 was obtained, which agrees with the values reported in the literature. Finally, the immersion heats of a commercial activated carbon, Carbochem PS‐30, into solutions with different concentrations of phenol (from 10 to 100 ppm) were measured. It was found that there is a direct relationship between the immersion heats and the concentrations of the phenolic solutions. The immersion heats are between 15 and 36 J g−1 of phenol.