In this manuscript we present, in detail, the mechanism of the selective dissolution of calcite from low-grade phosphate ores (Epirus area, Greece) by dilute acetic acid. It was found that mass transfer makes a significant contribution to the rate-determining step of the overall reaction of four separate pH regions discriminated within the pH range from 3.96 to 6.40. Moreover, two different reaction mechanisms were proposed and two different model equations best fitted the corresponding experimental data. By applying a particular treatment to the developed differential equations proposed by us earlier (see Ref. (2)), we estimated the rate constants of specific slow reactions referred to pH regions 3.96-4.95 and 4.95-5.50. However, for pH region 5.50-6.40, we cannot reach a similar conclusion so far.