We investigated potassium exchange equilibria in 20 calcareous soils from Iraq. Soils were varied in their CEC and miner-alogical composition. The percentage of CaCO3in these soils ranged from 10 to 29. The mean (Ca) (CO3) ion activity product (IAP) was found to be 5.89 x 10−8(pIAP = 7.23) with the standard deviation of 0.391 x 10−8. The experimental data on EKR and KAR exhibited a constant Gapon exchange selectivity coefficient (KG) for EKR values less than 0.11, provided corrections for ion-pairing and complex formations have been made. However, the agreement between the data and the Temkin equation was closer than that of Gapon equation. The plot ofKGversus pIAP did not reveal a definite relationship between these quantities.