The enzyme kinetics of an immobilized arylsulfatase reactor were examined. We found that the optimum operating conditions for the reactor were pH 7.0 and 25°C, using p-nitrophenyl sulfate in acetate buffer. The Michaelis constant (Km) of immobilized arylsulfatase was 5.29 mM, compared with a Km of 2.18 mM for soluble arylsulfatase from the same source (Helix pomatia). Since arylsulfatase hydrolyzes organic ester sulfate linkages, the immobilized arylsulfatase reactor was used to examine ester sulfate compounds in two soils subjected to different fertility management schemes. Soil samples were obtained from the Ap horizons of a Podzol from S-amended wheat plots and a Regosol from dykeland hayfield plots which had received additions of NH4NO3 and compost. The distribution of S in these soils was examined in the fall of 1993 and the spring of 1994. Soil organic matter was extracted and separated into three molecular weight fractions ( 10 000). There was no difference in the ester sulfate content for the >10 000 fraction of control and S-amended Podzol soils; however, the S-amended samples had significantly higher quantities of hydrolysable ester sulfates than controls for the 500–10 000 range, indicating that S amendments resulted in the incorporation of ester sulfate into this lower molecular weight fraction. Both control and NH4NO3 treatments to the Regosol showed significantly higher quantities of hydrolysable ester sulfates in the >10 000 fraction, while compostamended plots showed no difference between the >10 000 and 500–10 000 fractions due to suspected microbial degradation of high molecular weight organic S compounds in the compost. Since there was no significant effect of sampling time, this study indicated that naturally occurring low molecular weight ester sulfate compounds accumulate in soil and persist during storage. Hydrolysable ester sulfates constituted 35–55% of the hydriodic acid-reducible S in these different soils and probably represent an important and easily mineralizable portion of total ester sulfates.