Low Molecular Weight Substances (LMWS), such as amino acids, sugars, and organic acids are released into soil by root exudation or plant and microorganisms residue decomposition. LMWS are subject to a variety of processes: microbial utilization and decomposition to CO₂, sorption by mineral particles and soil organic matter, or leaching. Although these processes lead to low concentrations of these substances in soil (about 1 µmol l^-1), LMWS are of great importance in the C- (and N-) cycle due to their high microbial availability contributing to their fast turnover. Despite sorption of LMWS on soil particles strongly reduces their microbial availability, LMWS sorption was investigated only in few studies. Most of them used concentrations higher than occur in soil, and none distinguished between sorption and decomposition simultaneously. We investigated sorption and decomposition of alanine in soil. Because of the low concentrations which have to be used in order to simulate natural conditions, we used ¹⁴C uniformly labeled alanine. The experiments were done with 1 g of dried, sieved, and re-moistened loamy soil. Alanine as representative for amino acids was added to soil as 2.5 ml aqueous solution and gently shaken for periods between 10 s and 1 d. Concentrations in the soil solution varied from 0.1 to 1000 µmol l^-1. Radioactivity in the supernatant was measured after centrifugation at 1500 g and represented not absorbed alanine. Using CHCl₃, HgCl₂, or NaN₃ as sterilizing agents allowed separate determination of microbial decomposition and sorption to the soil matrix. After 200 minutes no further biodegradation could be observed. The soil-solution equilibrium was reached after absorption of about 50% of alanine within 150 minutes with 40% of the alanine absorbed already within the first 10 minutes. The sterilizing agents had various effects on alanine degradation. CHCl₃ prevents completely microbial mineralization, but was only partly suitable for sorption experiments because of release by cell lysis of similar substances which compete for sorption places. For this reason, a combination of HgCl₂ and NaN₃ is recommended. Without any sterilizing agents only 20% of the alanine remained in the solution. Consequently, at least 30% of the alanine were microbially decomposed within 3 hours. Comparing sorption of alanine and decarboxylated alanine we conclude, that the sorption of amino acids is mainly controlled by amino-group. At soil solution concentration of 1 µmol l^-1 alanine sorption of about 0.7 µmol kg^-1 soil for both, alanine and decrboxylated alanine was observed. The desorption experiment was carried out in two steps, first with deionised water, then with 0.01 M CaCl₂ solution. Desorption with water showed that about 10% of the sorbed alanine were absorbed only slightly, whereas two thirds of this absorbed amount were decomposed in non-sterilized soil. Subsequent desorption with CaCl₂ showed that another 10% of absorbed alanine were absorbed at cationic exchange places, with 80% of this amount decomposed in the non-sterilized variant. We conclude that sorption of alanine is faster than its microbial decomposition and that this sorption is irreversible.