A detailed mechanism of the selective dissolution of calcite from low-grade phosphate ores (Epirus area, Greece), using dilute acetic acid, and a model equation which fitted the experimental data perfectly, are presented in this report. The model equation has the form y = C-1.e(r1.t) + C-2.e(r2.t) Of a double exponential decay, where r(1) and r(2) are relations of the rate constants k(2) and k(4) of the slow reactions in the dissolution mechanism. By suggesting some reasonable postulates based on this work, we estimated the values of k(2) and k(4) from r(1) and r(2); consequently, we deduced that the transformation of H2CO3 to CO2 (gas) is most likely to be the rate-controlling step of the overall mechanism. Finally, the analysis of our experimental data according to three different tests showed that the controlling step of this heterogeneous reaction is a chemical change.