In the first part contours of the second order rate constant (k2) for homogeneous aromatic nitration have been determined over a portion of the three-component diagram (H2SO4-HNO3-H2O) using three aromatics of differing reactivity (toluene, chlorobenzene and p-dichlorobenzene). Activation energy measurements are also reported over the same section of the three-component diagram. Those results lead to the conclusion that the major factor in rate and energy considerations is the acidity of the medium and that some rates previously measured in heterogeneous systems are kinetically controlled.