Solid phase microextraction (SPME) was used to investigate the sorption behavior of ionogenic and non-ionogenic alkylated organotin compounds onto humic organic matter (HOM). The basic idea of these investigations is to measure selectively the freely dissolved proportion of the target analytes rather than the proportion bound to the polymer. This allows partition coefficients of non-ionic organic chemicals to be determined without any disturbance of the sorption equilibrium in both particulate and dissolved HOM matrices. For SPME, the ionogenic organotin compounds have to be alkylated. This can easily be achieved after centrifugation of particulate HOM, whereas for SPME in the presence of dissolved HOM an in-fiber derivatization is proposed. The sorption coefficients of a given ionogenic or non-ionogenic organotin compound are similar for particulate and dissolved HOM from the same source, indicating that the same type of interaction accounts for the sorption process. The higher the degree of alkylation in the target analyte, the more pronounced the nonspecific interactions and the faster the sorption equilibrium are established. Kinetic studies performed by SPME indicate that the sorption equilibrium on dissolved HOM, is reached within a few minutes; on particulate HOM the process takes longer due to diffusion pathways.