The so-called Modified Martempering discussed in this work differs from the standard martempering by that the temperature of the quenching bath is below the Ms point. In spite of the fact the lower temperature increases the severity of quenching, this also usually avoids the bainite formation, and by this reason, it is possible to make a fair comparison between different processes, which result in different microstructures. The present study shows the results in terms of mechanical properties, impact resistance in special of a cold work tool steel class, after being heat treated by the isothermal modified martempering process, as well as a comparison with the conventional quenching and tempering process and the austempering as well. A study to understand the bainitic microstructural transformation evolution has also been accomplished, gradually increasing the time of permanence in the bainitic transformation zone, verifying the gradual increase of the bainite fraction until the total transformation. At the end, microhardness and impact tests have been performed even in the austempering, modified martempering and tempered, and quenching and tempered samples, making possible to check the interaction between microstructure and impact toughness at the same hardness range.