ABSTRACT This paper studies whether firms opportunistically make proprietary claims in mandatory environmental disclosure programs with trade secret exemption rules. Examining the mandatory chemical disclosure program in the fracking industry, I find evidence of opportunistic withholding of information among operators that are less likely to have trade secrets. Specifically, I find that these operators claim fewer chemicals as trade secrets when the operating site is in close proximity to water quality monitors. This is only observed among publicly traded operators that face a higher cost of societal backlash when disclosing pollutant information. Further analyses suggest that these operators are concerned about external environmental monitoring, which deters them from opportunistic information withholding. Regarding public and private operators that are more likely to have trade secrets, I do not find strong evidence that their information withholding varies with the monitoring conditions.