The use of stable carbon and oxygen isotopic compositions (δ¹³C and δ¹⁸O) of bivalve shells has received growing attention in recent years in detecting ocean acidification. Bivalve shells are composed of calcium carbonate (CaCO₃), and exist in polymorphism as aragonite and calcite. Although polymorphs have identical chemical composition, they have differences such as in bioavailability, stability, and physical structure. In this study, We review the theory and practice of using stable isotopic tools in detecting ocean acidification, and report an example of using scallop shells from the Qualicum Beach of the Vancouver Island, British Columbia, Canada. The life-time δ¹⁸O values from a 1993 shell showed three peaks and valleys continuously, indicating about 3-yr life history. The δ¹³C values of the same shell showed isotopic variations from -0.5 to +0.75‰ over the same growth period. As compared with a 2014 shell, the latter indicated a drastic increase in δ¹⁸O and a stable decrease in δ¹³C values in the second summer: during which the Island Scallop company had to scale operations back considerably. These results suggest that scallop shells are good proxies for reconstructing the life history and environmental changes that the animal experienced (mainly from δ¹⁸O); and the isotopic data of shell carbonate have the potential to examine the effects of ocean acidification (mainly from δ¹³C) in the past.