The stoichiometry of the reaction between ground granulated blast furnace slag (GGBFS) having an empirical formula of C(7.88)S(7.39)M(3)A and calcium hydroxide (CH) was investigated. Scanning electron microscopy (SEM) was used to determine the slag consumption as well as the Ca/Si ratio in calcium silicate hydrate get (C-S-H) products. A tentative stoichiometric ratio of 2.6 mol of CH consumed per mole of slag reacted was determined using two methods. By combining consumption data determined separately for slag and CH a molar stoichiometry of 2.79 was found. Similarly, by directly determining the Ca/Si ratio in the C-S-H gel product, a range for the molar stoichiometry between 1.65 and 3.42 was found. Finally, a comparison of the stoichiometry of the slag/CH reaction was made with slag/portland cement hydration. The basic features of both appear similar. In the C-S-H gel around slag grains, a calcium-to-silica ratio of 1.3 to 1.4 was found for both slag/CH and slag/cement systems.