Estuarine waters are more susceptible to acidification because they are subject to multiple acid sources and are less buffered than marine waters. Consequently, estuarine shell-forming species may experience acidification sooner than marine species although, the tolerance of estuarine calcifiers to pH changes is poorly understood. This study analyzed 23 years of Chesapeake Bay water quality monitoring data and found that daytime average pH significantly decreased in moderately brackish waters, but pH has not significantly changed in less brackish waters. In some tributaries that once supported large oyster populations, pH is increasing. Current average conditions within some tributaries, however, correspond to values that in laboratory studies reduced oyster biocalcification rates or resulted dissolving of the shell. In a laboratory experiment, calcification rates of juvenile eastern oysters decreased when pH dropped by 0.5 units, but higher temperature and salinity mitigated the decrease in calcification.