The increasing usage of Positron Emission Tomography (PET) has resulted in the commissioning of new cyclotron facilities. Emissions from exhaust ducts at these facilities are continuously monitored for radioactivity in order to ensure compliance with regulations. A primary standard for positron emitters in gas is essential to ensure the accuracy and traceability of these measurements. Up to now, all the primary standardisations for positron emitters have been carried out on liquid or solid sources; there is no primary calibration facility for the instrumentation used to monitor the radioactive effluent from the PET/cyclotron sites. A system of gas flow proportional counters, already established at NPL for calibration of b ร emitters, will be the basis of the primary standard. This paper focuses on the calculation of positron energy deposition in proportional counters, and how this differs from electron energy deposition in order to predict the counters response. The PENELOPE Monte Carlo code has been used to study the energy deposition of positrons arising from the radionuclides 11 C, 13 N, 15 O and 18 F in a P10 gas for each of the three counters, and how these differ from energy depositions of electrons of the same energy at a pressure of 725 mmHg and temperature of 20.2 1C. The positrons were found to deposit from around 1.5 to 2.5 times more energy than electrons to the gas per primary shower depending on the maximum energy of the positron, affecting the minimum detectable activity concentration limit of the counters to positrons.