Ionization chambers (ICs) are the most commonly used instrument for measuring activities of solutions containing radiopharmaceuticals used in nuclear medicine, due to their simplicity of operation and accurate measurement results. As the dose delivered to the patient is determined by these measurements, for optimal results, the activity of these radiopharmaceuticals must be determined as accurately as possible, so that the radioprotection requirements are accomplished and at the same time good therapy or imaging results are achieved. In this work, the response of the ionization chamber Curiementor 2, from PTW-Freiburg, was studied by simulation with Monte Carlo (MC) code MCNPX and compared with experimental results for several radionuclides. The good agreement between experimental and simulation results validates the used methodology. The energy dependent sensitivity function, S (E), was established. Several ionization chambers were studied for (18)F and (99m)Tc, through Monte Carlo simulation method. In particular, some alterations to the original design of the Curiementor 2 were analysed, as well as the geometry of more recent chambers developed by PTW, namely Curiementor 3 and PET Curiementor 4. Calibration coefficients were compared for sources at different positions. Importance volumes were defined and determined for the Curiementors 2 and 3 and its ratio related with the corresponding ratio of sensitivity functions.