Iron chlorosis is one of the most important nutritional problems observed in plants, especially in ornamental and fruit trees, grown in calcareous soils. In Isfahan, presence of severely chlorotic plane trees ( Platanus orientalis ) undesirably affects the streets and parks views. The purpose of this study was to find the reasons of iron chlorosis in plane trees. During the years 1387 and 1388, 73 plane trees in different regions of Isfahan city with different soil properties and chlorosis intensities were selected and young and mature leaves were sampled from each tree. Activity of glutathione peroxidase (GPX) and Fe, Cu, Zn, carotenoids and chlorophylls concentrations were determined in mature and young leaves from both sampling years. Soil sampling of 0-30, 30-60 and 60-90 cm depths was also performed and soil properties such as pH and EC, as well as concentration of HCO 3 - , Cl - , Ca, Mg, K, Na, Fe, Cu, Zn and Mn were determined. The results didn't confirm any relation between iron concentration and chlorosis intensity of the leaves. In the first year, there was a significant relation between Fe and Zn concentrations in young leaves. Iron concentration in the mature leaves had a significant relation with GPX enzyme activity. There was also a negative relation between Fe concentration in mature leaves and EC and Cl - concentration in 30-60 cm depth of soils. Iron concentration of young leaves sampled in the second year had significant relation with leaf Zn and Cu concentrations. There was also a relation between Fe concentration of mature leaves and GPX enzyme activity in the second year of study. Multiple regression analysis showed that leaf P concentration had the most negative effect on Fe concentration in mature leaves of the second year. In both years, P concentration and P/Fe ratios of young leaves was higher than those of the mature leaves. The GPX enzyme activity was strongly related to Fe concentration in mature leaves that shows the important role of Fe in GPX enzyme activity. It is suggested that P/Fe ratio and GPX activity be used for diagnosing of Fe chlorosis of plane trees. The mean concentration of available Fe in the soils supporting the trees with most severe intensity of chlorosis was less than those supporting the trees with low intensity of chlorosis. There was no relation between Fe concentration in plane tree leaves and available Fe in soils or soil pH values. In sampling areas with severely chlorotic trees, bicarbonate concentration in 0-30 cm of soil depth was more than those with gently chlorotic trees. Based on the results of this research, it seems that the main reason of plane tree chlorosis in Isfahan green-space is deficiency of available Fe in soil and low Fe activity in leaves as a result of high Cu and P concentration. The GPX activity and a and b chlorophyll concentrations can be used as indicators for diagnosing of Fe chlorosis