Plants are important bio-indicators for environmental pollution. This study was conducted to assess trees as indicators of environmental pollution near Bahrain oil refinery. Four tree species were located near the refinery were used in this study; Neem (Azadirachta indica), Nerium oleander, Phoenix dactylifera (Date palm) and Conocarpus erectus. The same set of trees, located near Arabian Gulf University were used as control. Leaf samples were collected monthly from the trees and analyzed for heavy metals, sulfur, chlorophyll and water content. Results indicated that Air Pollution Tolerance Index (APTI) tend to increase in trees grown in the polluted site, with Azadirachta giving the highest APTI followed by Nerium. Chlorophyl content is lower in polluted site. Nerium and Conocarpus significantly accumulated more Chromium than Date palm and Conocarpus in the polluted site. Nerium accumulated more Cu, Cr, Mn, Mo and Fe than the rest in the polluted site. No differences in Cd and Pb leaf content between tree species or between sites. Sulfate content was higher in the study site compared to the control site in all plant species Azadirachta leaves accumulated more sulfates followed by Date palm Nerium and Conocarpus. This study indicated that trees remove significant amounts of pollutants from the environment and are excellent bio monitors of pollution stress.