Two new reactivity indices were developed within the conceptual framework of the Density Functional Theory. These new descriptors, called electrophilic susceptibility index and nucleophilic susceptibility index, were tested to reproduce the chemical reactivity trends of series of halogen-substituted quinones and 5-substituted indoles, respectively. The above mentioned molecular system were computationally treated by using the B3LYP/6-31G(d) level of theory. For these systems, the proposed descriptors showed to be more appropriate than other descriptors currently known such as the electrodonating power, the electroaccepting power and the popular electrophilicity index.