Abstract: Process for the production of 2,4,5,6-tetrachloroisophtalonitrile, with formula (I), having a crystallographic structure suited for producing fungicide compositions remaining stable throughout time and having a high biological activity, wherein a thermal and/or mechanical treatment is carried out on the non-suited crystallographic structure, both by gradually decreasing the temperature during the de-sublimation phase, subsequent to industrial synthesis, and by supplying thermal energy--in static or dynamic conditions--and/or mechanical energy to the industrial technical product having a nonsuited crystallographic structure eventually during the phase of preparation or of micronization of said product.