The calcium carbonate (CaCO₃) and magnesium carbonate (MgCO₃₎ contents of the periwinkle (Tympanotonus fuscatus and Littorina littorea) shell found in Uyo, Akwa Ibom State were determined by complexometric titration. Results indicated that Tympanotonus fuscatus and Littorina littorea had high level of CaCO₃ (88.22 ± 0.75% and 85.38 ± 0.80% respectively) and a relatively low but substantial level of MgCO₃ (10.25 ± 0.42% and 9.43 ± 0.60% respectively). The result is discussed with particular reference to the suitability of the periwinkle shell being used as slurry in thin layer chromatography.