This study was conducted to characterize the reactivity of carbonates in the surface horizons of 20 soils (Entisols and Aridisols) from central region of Iran. The samples were analyzed for Calcium Carbonate Equivalent (CCE), Active Calcium Carbonate Equivalent (ACCE), Non-active Calcium Carbonate Equivalent (N.ACCE), carbonate mineralogy and surface area of carbonates (SSA) through N2 adsorption. The CCEs ranged from 38 to 228g kg-1 of soil, with a mean of 105.1g kg-1 of soil. ACCEs1 (while not being sieved through 1mm sieve) ranged from 9.9 to 90.5 with a mean of 32.7gkg-1 of soil and ACCEs 2 (after being sieved through 1mm sieve) ranged from 13.4 to 99.8 with a mean of 37gkg-1 of soil. About 32% of the calcium carbonate (without being sieved through 1mm sieve) and about 34% of CaCO3 in the soils reacted with oxalate. The average surface area of soils as estimated from the N2 adsorption (BET) was 19.75 m2 g-1 of soil and the average surface area of carbonates was 103.2 m2 g-1 of calcium carbonates. The surface area of carbonates was significantly correlated with clay content (r = 0.46*, n=20) and with CCE (r=-0.52*, n=20), indicating that with increase in CCE content, the surface area decreases. X-ray diffraction (XRD) analysis showed that Mg- Calcite [Mgx Ca1-x (CO3)] is the most commonly cfound carbonate in the studied soils. Dolomite was identified in only 7 soils.