Effects of L-aspartic acid (L-Asp) on dissolution of calcite were investigated. The step retreat velocity and dissolution rate of calcite were measured simultaneously using an AFM flow-through system. The etch-pit morphology of calcite was observed using confocal laser scanning microscopy. Results show that the etch-pit morphologies changed drastically depending on the L-Asp concentration ([L-Asp]) in the order of rhomboidal, pentagonal, and triangular (not perfectly, but retaining an extra step). The change in obtuse step directions and appearance of the 10 1 01 step triggered these morphological changes. Addition of L-Asp accelerated all step retreats at [L-Asp] <0.01 M, which implied the effect of L-Asp on the diffusive barrier. In contrast, at [L-Asp] > 0.01 M, L-Asp inhibited the retreats of obtuse steps and [0 1 0] step, although the retreat velocities of acute steps were constant irrespective of [L-Asp]. These results suggest that the directional changes and the inhibition of retreat velocities of obtuse steps were attributed to the generation of [4 1 1] and [4 5 1] steps caused by L-Asp. Moreover, we confirmed the preferential effects of L-Asp on the [4 8 1](+) to [4 4 1](+/-) step edge, and proposed the preferential effects of L-Asp on the [4 1 1] to [4 5 1] step edge. (C) 2010 Elsevier B.V. All rights reserved.