In the chemical etching process or chemical development is very important to know the track etching parameters, such as bulk etching velocity (Vb) and track etching velocity (V¡), to define the optimal time of etching. In this work both parameters has been experimentally measured in CR-39 that it was irradiated with a thermal neutron flux; the thermal neutrons produce protons by the nuclear reaction (n,p) with the air. The CR-39 detectors were chemically etched in sodium hydroxide 6,25 •N at 70 °C for time periods from 1 to 6 hours. The obtained results show that the value ofVt is almost twice Vb.