Technical Abstract: Aminocyclopyrachlor (DPX MAT28) a newly discovered synthetic auxin herbicide and its methyl ester (DPX KJM44) appear to control a number of perennial broadleaf weeds. The potential volatility of this new herbicide and its methyl ester were determined under laboratory conditions and were also compared to dicamba and aminopyralid using enclosed chamber and open air plant bioassays. Bioassays consisting of visual estimates of epinastic responses and kidney bean and soybean leaf width measurements were developed to measure vapor release from glass and plastic. Vapor release of aminocyclopyrachlor from glass surfaces was undetectable under laboratory conditions, and no phytotoxic responses were observed when plants were exposed to vapors emanating from various surfaces. Results were similar with aminopyralid indicating the risk of plant injury from vapor movement of aminocyclopyrachlor and aminopyralid was very low. When combined with 1% MSO, vapor release of aminocyclopyrachlor methyl ester reached 86%192 hours after application to glass surfaces. Phytotoxic responses of plants exposed to vapors emanating from various surfaces treated with aminocyclopyrachlor methyl ester were similar to responses to dicamba under enclosed incubation conditions but were less in outdoor, open-air environments. Studies are needed to better understand the risk of injury to non-target plants due to vapor movement of aminocyclopyrachlor methyl ester under field applications.