Objective: The Environmental Index (EI) signifies a material's resistance to damage resulting from organic liquids likely to be present in the mouth. This study determined the EI of a spectrum of resin-based restorative materials. Methods: The materials evaluated included an ormocer (Admira [AM]), a giomer (Beautifil [BF]), two compomers (Compoglass F [CF], Dyract Extra [DY]) a minifill (Esthet X [EX]) and a nanofill (Filtek Supreme [FS]) composite. Eight specimens were made for each material-medium combination. After light polymerization, the materials were preconditioned in Fusayama's artificial saliva (AS) at 37 degree C for 24 hours. The materials were then stored in the following food-simulating liquids at 37 degree C for 1 week: heptane (HP), 75% ethanol-water solution (ES), 50%ES, 25%ES, and distilled water (DW). Specimens stored in AS were used as control. After conditioning, the materials were subjected to microhardness testing (load = 500gf; dwell time = 20secs) and SEM evaluation. Hardness ratios were plotted against the solubility parameter of the solutions and the EI was subsequently calculated. Data were subjected to ANOVA/Scheffe (p<0.05). Results: For all materials, conditioning in HP resulted in significantly higher hardness values, while exposure to ES resulted in significantly lower values than conditioning in AS and DW. Ranking of EI was: BF > EX > FS > AM > DY > CF. SEM examination indicated that the organic solvent degradation may be associated with matrix cracking, filler dislodgement and coupling agent disintegration. Conclusions: The giomer was most resistant to damage by organic liquids while the compomers were the least. This research was supported in part by Grant R224000014112 from the Faculty of Dentistry, National University of Singapore.