Solution extraction and analysis in the first 3 h of hydration have been used to study the effects of calcium chloride, calcium nitrate, calcium thiocyanate, and sucrose on the hydration of Portland cement. Studies were made at a water : cement ratio of 0.5, which is similar to that to be expected in normal use. The addition of calcium salts has a pronounced effect on the calcium, hydroxyl and sulphate ion concentrations. This is in agreement with the results from X-ray diffraction and differential scanning calorimetry, which show rapid precipitation of more gypsum and calcium hydroxide within the first 5 min on adding these salts than in neat OPC pastes. The data for sucrose show opposite trends in line with the chelation of the calcium ion. No significant changes in sodium and potassium levels were observed in the first 3 h with any of the additions. Differences in degrees of hydration at early stages from the addition of calcium salts are attributed to different relative degrees of solvation of the ionic reactants and transition states, in addition to the nature and extent of sulphate and hydroxide formation from solution. The combined effect of cations and anions on the acceleration produced are discussed.