Studies by the US Salinity Laboratory, using Gapon rule, led to the value of 0.01475 (mmol L-1)-0.5for the modified Gapon selectivity coefficient (K'G). Nevertheless, the results of subsequent research reveled that K'G isn’t constant and could take on a wide range of values depending on soil properties. The aim of this study was to determine the relationship between Exchangeable Sodium Ratio (ESR) and Sodium Adsorption Ratio (SAR) in some salt-affected soils of Tabriz plain. To follow this, thirty composite soil samples were taken from the area. For the soils in the study area with SAR < 100 (mmol L-1)0.5 the ESR-SAR relationship was of the form: ESR=0.011+0.0072 SAR( r2=0.86) which didn’t follow the traditional USSL equation. After correction of the data for anion exclusion, this relationship was modified: ESR=-0.041+0.0120 SAR
(r2=0.90). As a result, taking into account the anion exclusion increased the K'G value considerably. However, K'G value was still lower than 0.01475(mmol L-1)-0.5.This may be attributed to the high salinity level of the investigated soils which can lead to an increase in the affinity of clay minerals for calcium and magnesium through other mechanisms (compression of DDL and/or the formation of quasicrystals) in addition to the anion exclusion mechanism. In a multiple regression procedure, ECe was negatively while ESP positively associated with K'G (r2=0.73). After correction of the data for anion exclusion the association was merely with ESP (r2=0.49).