Alcohol imposes significant social costs on the residents of Montana. The state has one of the highest alcohol-related traffic fatality rates in the nation, and alcohol accounts for more than one-eighth of deaths among working aged adults statewide. 24/7 Sobriety requires alcohol-involved offenders to abstain from alcohol and submit to frequent alcohol testing; those failing or missing a test face an immediate, but brief, jail term. The State of Montana began piloting 24/7 among driving-under-the-influence (DUI) arrestees in Lewis and Clark County in early-2010 and expanded to 22 counties with the passage of House Bill 106 in May 2011. The program also grew to include other alcohol-involved offenses, though DUI arrestees account for more than 75% of program participants. In Montana, 24/7 participants are monitored for an average of 160 days, with a median time of 104 days. Using data from everyone who was convicted of their second DUI charge (DUI-2) from January 2008 to August 2014, this analysis examines the effect of 24/7 participation on the probability of DUI re-arrest for participants within twelve months of their second (DUI-2) arrest date. Results from bivariate probit models which instrument with 24/7 availability to account for potential selection issues provide suggestive evidence that 24/7 participation reduced the probability of DUI re-arrest in Montana (perhaps on the order of 45% to 70% when considering both our main results and sensitivity analysis findings), but missing criminal history information for approximately half of the sample precludes us from making stronger inferences about causality.