This contribution describes a method for heat capacity determination in a small-scale reaction calorimeter under quasi-isothermal conditions (+/- 0.16 degrees C to +/- 0.77 degrees C). The heat capacity of the reactor content is calculated from the amplitudes and the phase shifts of the reactor temperature, of the electrical heater and of the cooling rate when forced temperature oscillations are applied. The heat capacities of eight solvents (water, ethanol, methanol, acetone, 1-octanol, diethylenglycol, toluene, and 1-butanol) covering a wide range of viscosity were calculated for various experimental conditions, including reactor volume and stirrer speed. Systematic deviations were detected when compared to the corresponding literature values. Straight line calibration with the total heat transfer coefficient and two modern multivariate calibration techniques (partial least squares and neural network) were applied to correct for these deviations. The different calibrations show similar precision and allow for an online determination of the heat capacity with an accuracy comparable to other published methods. Successful applications include the determination of the heat capacities for n-heptane, for various homogenous ethanol/water mixtures, and during the course of the hydrolysis of concentrated sulfuric acid. (c) 2008 Elsevier Ltd. All rights reserved.