The effect of modifiers on the hydration of cements was studied by isothermal calorimetry. Thermochemical experiments were carried out using a TAM III isothermal calorimeter (TA Instruments). For the TAM III calorimeter, a method was developed for conducting experiments on monitoring the thermal activity of solid-phase samples with the possibility of repeated use of steel ampoules with a volume of 20 ml. The processes of hydration of Portland cement grade CEM I 42.5B D0 LLC "Holcim Rus" without a modifier and with modifiers polycarboxylate type S (POLYPLAST LLC and «Glass Powder) were studied. Heat release thermograms were recorded, which showed good reproducibility of the results and their similarity with those available in the literature for similar studies carried out according to standard methods using a TAM AIR isothermal calorimeter (TA Instruments). This indicates the reliability of the obtained data and the possibility of using the method proposed in this article to study the processes of hydration of cements. The maximum heat release on the thermograms of samples with a polycarboxylate modifier is observed approximately 20 h after the start of heat release monitoring. Samples without the modifier and with the glass powder modifier show the maximum heat release approximately 10 h after the start of the experiment. Analysis of the results showed that the presence of modifiers leads to increased heat release during cement hydration in relation to its initial composition. Accordingly, it takes less time to complete hydration. However, it is also necessary to take into account the possible risks of violation of the technological modes of operation of the corresponding equipment due to excessive heat release during the hydration of modified cements.