An ecological study of soil diatoms on Ile de la Possession (Crozet Archipelago, sub-Antarctica) was carried out during the austral summer of 1998/1999. Both diatom and chemistry data were collected. A highly diverse diatom flora of 230 taxa, belonging to 39 genera were identified from 104 samples. Several of them are considered to be new. The most abundant genera were Diadesmis, Achnanthes and Pinnularia. Principal Component Analysis (PCA) was used to classify the samples based on their chemical characteristics. Moisture and nutrients were the main factors separating three groups of samples: dry fellfield soils, soils influenced by marine animals and mesic valley soils. Species assemblages correspond well with this division. Forward Selection with Monte Carlo permutations was used to reduce the number of significant variables. The analysis selected moisture, phosphate and sulphate concentrations. Based on the analysis, the same three groups of the PCA were found. Weighted averaging and calibration were used to develop a statistical transfer function to infer the moisture content of soils from the diatom assemblages.