Accumulation of heavy metals in soil can not only limit the plant growth and development but also may restrict the activity of soil microorganisms as well as soil extracellular enzyme activities. Soil enzyme activities are considered as sensitive indices to trace metal pollutions. Phosphomonoesterases are among soil phosphatases which catalyze the hydrolysis of organic phosphate esters to orthophosphates. They have been nominated as indices for evaluating the degree of Cd toxicity in contaminated soils. The objective of this study was to identify the interaction of CaCO 3 and Cd on the activity of phosphomonoesterases in acid soils. Two soil samples were collected from Leila Kouh, Lahijan from upper and lower parts of a slope. Both soils are acidic with contrasting textural properties. A preliminary experiment was performed to determine the optimum level of CaCO 3 application. Following the preliminary experiment two short- and long-term experiments were conducted to investigate the interactive impact of Cd and CaCO 3 application in the acid soils. To evaluate the degree of short- and long-term enzyme inhibition, inhibition percentage (IP) and ecological dose 50 (ED 50 ) were used, respectively. In the preliminary experiment CaCO 3 were applied at 0, 2, 8, 16, 32 and 64 g CaCO 3 kg-1 soil and four levels of 0, 2, 8 and 16 g CaCO3 kg-1were selected eventually. For the IP approach, the soils amended with CaCO 3 were applied with a rate of 6µmol Cd g-1 soil, incubated for 30 minutes, followed by the enzyme assays. For the ED 50 approach the CaCO3 applied soils were treated with a range of Cd concentrations (0, 4, 20, 100, 500 and 1000 mg kg-1), incubated for 65 days at 25 °C and 50% water holding capacity, followed by the enzyme assays. Two kinetic-based models (model 1 and 2) were fitted in the data to calculate the values of ED 50 . Results indicated that in both soils CaCO 3 application resulted in a proportional increase in the pH values. This was observed as greater application rates of CaCO 3 increased pH values. The CaCO 3 -induced pH increase, significantly enhanced the activity of acid phosphatase while, the activity of alkaline phosphatase decreased. The different responses of the enzyme activities were attributed to their different optimum pH. Interestingly, the IP values of Cd for both acid and alkaline phosphatase were significantly decreased. The IP values in the loamy sand soil decrease from 17.8 to 6.5%. Similarly the values diminished in the silty clay soil from 29.4 to 7.8%. Regardless of the increased values of alkaline phosphatase in the CaCO 3 -ammended soils, the IP values of alkaline phosphatase decreased due to CaCO 3 application. It was observed that the IP values of the loamy sand soil decrease from 21.0 to 6.4%. Similarly the values diminished in the silty clay soil from 21.6 to 6.2%. Regardless of the CaCO 3 level, Cd application increased the DTPA-extractable Cd from 0.2 to 566 mg kg-1. Calculation of ED 50 values by the two models revealed that even though both models could estimate the ED50 values, the correlation coefficients of the model 2 were generally higher. Expectedly, the model 2 conformed well to the data and revealed that Cd inhibition to the phosphomonoesterses obeys a partial rather than a complete inhibition mechanism by which it is anticipated that a protected part of the enzyme cannot be reached by the Cd ions. Overall, it can be concluded that CaCO 3 can cease the Cd inhibition and it is suggested that lime application can be thought as a short and inexpensive solution for managing the acid soils acutely exposed to elevated Cd concentrations. Keywords: Phosphomonoesterase, CaCO 3 , Cadmium, IP, ED