The conversion of grassland into cultivated land is a common agricultural practice, generally leading to the decrease of the soil organic matter (SOM) content. In this study, we analysed quantitative changes in carbon content. Additionally qualitative changes occurring in the soil organic matter composition on a molecular basis were assessed using Curiepoint pyrolysis coupled to gas chromatography and mass spectrometry (pyrolysis GC/MS). The aim of the study was to follow the development of SOM in grassland soil, after conversion into arable soil. Soil was sampled before the conversion (0 month) as well as 3 months, and 1 year after the conversion. The samples were treated with 10% HF to remove mineral material before being subjected to analysis of the bulk chemical composition by pyrolysis GC/MS. The relative contributions of single molecules were obtained by the integration of the total ion chromatogram. Pyrolysis products derived from lignins, proteins and polysaccharides were identified in all samples. SOM under grassland, arable land and converted grassland released similar pyrolysis products. Three months after the conversion, lignin-derived pyrolysis products were found at lower concentrations in the converted grassland soil. Principal component analysis showed that arable land, grassland and the converted grassland could be distinguished using the score plot of the 2nd and 3rd principal components. The differences induced by grassland conversion are only transitory and I year after the conversion, SOM has a similar composition as SOM of the initial grassland soil.