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Acid phosphatases belong to the group of enzymes, phosphomonoesterases, which play a major role in the mineralization of soil organic P. Substrate concentration of enzyme is one of the factors that affect the enzyme kinetics. To study the role of substrate concentration on soil enzyme activity in alfisols, twenty-five surface soil samples were collected and assayed for the activity of soil acid phosphatase. The acid phosphatase activity as expressed in terms of µg of 4-nitrophenol released g-1 soil h-1 in these soils varied from 16.9 to 268.4 with a mean value of 58.8 in surface soils. Among them, ten soil samples were selected to study enzyme kinetics with thirteen substrate concentrations under laboratory incubation studies with three replications. Soil acid phosphatase increased with an increase in substrate concentration up to 20mM and almost reached a plateau at a substrate concentration of more than 15mM. The enzyme activity remained almost constant in all the ten alfisols with further increase in substrate concentration, minimal change in enzyme activity was observed. Characteristics of enzyme activities like maximum enzyme reaction velocity (Vmax) and Michaelis constant (Km) were determined using the Michaelis-Menten equation similar to those determined in a homogenous system. The maximum reaction velocity of soil acid phosphatase for soils under study were calculated (µg of 4-nitrophenol g-1 soil h-1) and varied from 30.0 to 434.8 and the soils S6, S8, S9 and S10 recorded higher values using Line weaver-Burk plot. The values compared well with Hanes-Wolf transformation 25.5 to 364.7 and the soils S6, S8, S7 and S9 recorded higher values. Under Eadie - Hofstee transformation the values varied from 27.7 to 390.3 and higher values were recorded by S6, S8, S9 and S10. Michaelis constant (Km) of the soil acid phosphatase was calculated using Line weaver-Burk transformation plot, varied from 4.9 to 9.6 mM and the higher values were recorded by S6, S8, S1 and S10. The values compared well with those obtained from Hanes-Wolf 2.6 to 6.8 and soils S6, S8, S4 and S7 recorded higher values. In Eadie-Hofstee plots the values ranged from 3.7 to 7.8 and higher values were found in the soils S6, S8, S10 and S4. It was concluded that the linear transformation of Line-weaver Burk values was superior for estimating Km and Vmax values of enzymes in soil. However significant differences were noticed when Km and Vmax were calculated from the linear transformation of Michaelis plot. [ABSTRACT FROM AUTHOR]