Miniature fission chambers are suited tools for instrumenting experimental reactors, allowing online and in-core neutron measurements of quantities such as fission rates or reactor power. A new set of such detectors was produced by CEA to be used during the next experimental program at the E facility starting in 2013. Some of these detectors will be employed in pulse mode for absolute measurements, thus requiring calibration. The calibration factor is expressed in mass units and thus called “effective mass”. A calibration campaign was conducted in December 2012 at the SCK·CEN BR1 facility within the framework of the scientific cooperation VEP (VENUS-E-PROTEUS) between SCK·CEN, CEA and PSI. Two actions were conducted in order to improve the calibration method. First a new characterisation of the thermal flux cavity and the MARK3 neutron flux conversion device performed by SCK·CEN allowed using calculated effective cross sections for determining detectors effective masses. Dosimetry irradiations were performed in situ in order to determine the neutron flux level and provide link to the metrological standard. Secondly two fission chambers were also calibrated at the CEA CALIBAN reactor (fast neutron spectrum), using the same method so that the results can be compared with the results obtained at the SCK·CEN. In this paper the calibration method and recent improvements on uncertainty reduction are presented. The results and uncertainties obtained in the two reactors CALIBAN and BR1 are compared and discussed.