The microbial degradation of the water-soluble components of gas oil was measured both qualitatively and quantitatively. The mixed autochthonous flora in clean ground water served as the inoculum in continuous percolation experiments. Concentration gradients of individual compounds as well as of DOC were measured as a function of the percolation time. A lag period of ∼5 days at 10°C and of ∼1 day at 25°C respectively preceded measurable attack of the dissolved hydrocarbons. At the experimental infiltration rates, complete hydrocarbon elimination within a few decimeters of percolation depth occurred when steady state conditions of the microbial settlement of the sand were established.