The Swiss Soil Monitoring Network NABO records the long-term evolution of soil organic carbon (SOC) over extended periods. Two different analytical methods were used to determine SOC, namely wet oxidation (WO) and retitration of potassium dichromate (Swiss Standard Method) and dry combustion (DC) with a CN-analyser and subtraction of inorganic carbon. Generally, the WO method generates lower SOC contents than the DC method. Therefore, SOC contents by WO must be adapted to the level of the DC method to avoid distortion of temporal evolutions. For NABO soils, conversion factors between the two methods ranged from 1.01 to 1.42 with a median of 1.18. Due to the large variation between sites, site-specific conversion factors should be used, whenever possible.