We report here a new method for the determination of electron capture (EC) rate constants that utilizes a pulsed electron beam mass spectrometer. The method is first tested by measurements of the known dissociative electron capture rate constants for several halogenated methanes that have been extensively studied by other techniques. The resonance electron capture (REC) rate constants of nitrobenzene (NB) and 23 substituted nitrobenzenes (SNB’s) are then determined for the first time at 125 degrees C in 10 Torr of methane buffer gas. The SNB’s studied here include several sets of closely related structural isomers whose electron affinities (EA’s) have been previously determined. It is shown that the REC rate constants of these compounds bear little systematic relationship with the EA’s of these compounds. The REC rate constants of the SNB’s are also compared with other previously reported characteristics associated with the negative ionization of these compounds, including their entropies of negative ionization, the lifetimes against autodetachment of their initially formed molecular anions, and the rates of autodetachment from electronically excited states of their molecular anions.