Genetically tractable model plants offer the possibility of defining the plant O(3) response at the molecular level. To this end, we have isolated a collection of ozone (O(3))-sensitive mutants of Arabidopsis thaliana. Mutant phenotypes and genetics were characterized. Additionally, parameters associated with O(3) sensitivity were analysed, including stomatal conductance, sensitivity to and accumulation of reactive oxygen species, antioxidants, stress gene-expression and the accumulation of stress hormones. Each mutant has a unique phenotypic profile, with O(3) sensitivity caused by a unique set of alterations in these systems. O(3) sensitivity in these mutants is not caused by gross deficiencies in the antioxidant pathways tested here. The rcd3 mutant exhibits misregulated stomata. All mutants exhibited changes in stress hormones consistent with the known hormonal roles in defence and cell death regulation. One mutant, dubbed re-8, is an allele of the classic leaf development mutant reticulata and exhibits phenotypes dependent on light conditions. This study shows that O(3) sensitivity can be determined by deficiencies in multiple interacting plant systems and provides genetic evidence linking these systems.