Heat capacity for 1-butyl-3-methylimidazolium tosylate [C(4)mim][Tos] in the temperature range (5 to 370) K has been measured by adiabatic calorimetry. Temperatures and enthalpies of its phase transitions have been determined. Thermodynamic functions have been calculated for crystalline and liquid states. The thermal stability of [C(4)mim][Tos] has been determined by scanning calorimetry. Mutual solubility in binary mixtures of [C(4)mim][Tos] with water and caprolactam has been investigated, and the activity coefficients have been calculated. Heat capacities for three mixtures of [C4MiM][Tos] with water have been measured by adiabatic calorimetry. On the basis of the calorimetric and SLE measurements for ([C(4)mim][Tos] + water) samples the heat capacity anomalies have been interpreted.