We exploit two regulatory shocks to examine the informational effects of tightening preexisting mandatory disclosure rules. Canadian National Instrument 51-101 in 2003 and the U.S. rule “Mod-ernization of Oil and Gas Reporting” in 2009 introduced quasi-identical amendments which effec-tively tightened the rules governing oil and gas reserve disclosures in both countries. We document signiﬁcant changes in ﬁrms’ reporting outcomes when the new regulations are intro-duced. We also ﬁnd that the reserve disclosures ﬁled under the new regulations are more closely associated with stock price changes and with decreases in bid-ask spreads. Our ﬁndings are robust to controlling for other confounding factors such as time trends, other information disclosed simul-taneously, ﬁnancial reporting incentives, mispricing, and monitoring efforts.