Along with some research networking programmes , the European Directive 2008/50/CE requires chemical speciation of fine aerosol (PM 2.5), including elemental (EC) and organic carbon (OC), at a few rural sites in European countries. Meanwhile, the thermal-optical technique is considered by the European and US networking agencies and normalisation bodies as a reference method to quantify EC-OC collected on filters. Although commonly used for many years, this technique still suffers from a lack of information on the comparability of the different analytical protocols (temperature protocols, type of optical correction) currently applied in the laboratories. To better evaluate the EC-OC data set quality and related uncertainties, the French National Reference Laboratory for Ambient Air Quality Monitoring (LCSQA) organised an EC-OC comparison exercise for French laboratories using different thermal-optical methods (five laboratories only). While there is good agreement on total carbon (TC) measurements among all participants , some differences can be observed on the EC / TC ratio, even among laboratories using the same thermal protocol. These results led to further tests on the influence of the optical correction: results obtained from different European laboratories confirmed that there were higher differences between OC TOT and OC TOR measured with NIOSH 5040 in comparison to EUSAAR-2. Also, striking differences between EC TOT / EC TOR ratios can be observed when comparing results obtained for rural and urban samples, with EC TOT being 50 % lower than EC TOR at rural sites whereas it is only 20 % lower at urban sites. The PM chemical composition could explain these differences but the way it influences the EC-OC measurement is not clear and needs further investigation. Meanwhile, some additional tests seem to indicate an influence of oven soiling on the EC-OC measurement data quality. This highlights the necessity to follow the laser signal decrease with time and its impact on measurements. Nevertheless, this should be confirmed by further experiments , involving more samples and various instruments, to enable statistical processing. All these results provide insights to determine the quality of EC-OC analytical methods and may contribute to the work toward establishing method standardisation.