Protection of soils against contamination is a major principle of long-run productivity and sustainable agriculture. Heavy metals including Cd negatively influence the soil community structure and microbial activities and hence, affect soil health. Soil enzyme activities have been frequently used as soil bio-indicators of soil quality. Phosphatases play a vital role in P mineralization and consequently in plant nutrition. The enzymes are also adversely affected by salinity. Ecological Dose 50 (ED 50 ), the concentration that results in a 50% reduction in a biological function has been used as a quantitative index of toxicity. The objective of this study was to study the additive effects of salinity on Cd toxicity on soil alkaline phosphatase. In order to study the effects of salinity on ED 50 and Inhibition percent of Cd on soil alkaline phosphatase two contrasting soils were sampled. Soil samples include a calcareous (Shervedan) and an acidic (Langroud) soil. Soils were air-dried, passed through a 2-mm sieve and were artificially salinized by NaCl to achieve electrical conductivities 13, 18 and 28 dS m -1 . The soils then were contaminated with varying concentrations of Cd 0, 3, 10, 50, 100, 300, 700, 1000 and 5000 mg Cd/kg. Soils were incubated at 25 °C and 50% water holding capacity for 3 and 60 days and at the end of the incubation period, soil alkaline phosphatase was determined by determining p -nitrophenol released. Furthermore, NH 4 NO 3 -extractable Cd was also measured by atomic adsorption spectroscopy. Two kinetic models (models 1 and 2) and a sigmoid dose response curve (model 3) were fitted to the data. Results indicated that Langroud soil had significantly greater Cd concentrations. Salinity enhanced the extractable Cd concentration. Longer incubation period increased the extractable Cd concentration and resulted in lower alkaline phosphatase activity. The enzyme activity significantly decreased as the applied Cd concentration increased. A more pronounced reduction in the enzyme activity was observed as salinity increased. In Shervedan soil, kinetic based models 1 and 2 were satisfactory for 60 and 3 days of incubation, respectively. The ED 50 values calculated by models 2 and 3 were decreased as the salinity increased. In lordegan soils, neither of the models could calculate ED 50 values. The inhibition percent was consistently greater in Langroud soil and enhanced in the both soils as soils were salinized. Overall, we found out that Cd ia a limiting factor for P mineralization in soils and salinity can enhance the toxicity of Cd on alkaline phosphatase. This finding supports the hypothesis suggesting that salinity additively increase the Cd toxicity on soil alkaline phosphatase.