Calcium (Ca) is an important element for neutralizing soil acidity in temperate forests. The immediate availability of Ca in forested acid soils is largely dependent on mineralization of organic Ca, which may differ significantly among tree species. I estimated net Ca mineralization in the forest floor and upper 15 cm of mineral soil beneath six tree species in a mixed-species forest in northwestern Connecticut, using the buried bag method. Net Ca mineralization in the forest floor was significantly correlated with mass loss of the decomposing forest floor litter. Higher mass loss fractions during the summer and in forest floors beneath sugar maple (Acer saccharum) and white ash (Fraxinus americana) coincided with higher net Ca mineralization rates. More Ca was released per unit mass loss of forest floor beneath sugar maple and white ash (362 and 390 mmol kg−1, respectively) than beneath American beech (Fagus grandifolia), red maple (Acer rubrum), red oak (Quercus rubra) and hemlock (Tsuga canadensis) (183, 133, 147, and 190 mmol kg−1, respectively). Due to the high forest floor mass beneath red maple, beech, red oak, and hemlock, net Ca mineralization in the forest floor per unit area beneath these tree species did not differ significantly from sugar maple and white ash (ranging between 80 mmol m−2 per year for beech and 141 mmol m−2 per year for sugar maple). Net Ca mineralization in the mineral soil was significantly larger beneath sugar maple (142 mmol m−2 per year) than beneath the other tree species (ranging between −10 mmol m−2 per year for beech and 55 mmol m−2 per year for white ash). These results show that Ca mineralization rates are large and differ significantly among tree species, affecting the spatial pattern of soil acidity and Ca availability in a mixed-species forest stand.