Organic peroxy radicals ("RO2HTC/SUBTAG"FORTITLEHTC_RETAIN, with R organic) are key intermediates in most oxygen-rich systems, where organic compounds are oxidized (natural environment, flames, combustion engines, living organisms, etc). But, until recently, techniques able to monitor simultaneously and distinguish between RO2 species ("speciated" detection) have been scarce, which has limited the understanding of complex systems containing these radicals. Mass spectrometry using proton transfer ionization has been shown previously to detect individual gas-phase RO2 separately. In this work, we illustrate its ability to speciate and monitor several RO2 simultaneously by investigating reactions involving CH3O2, CH3C(O)O-2, c-C6H11O2, and (CH3)(3)CO2. The detection sensitivity of each of these radicals was estimated by titration with NO to between 50 and 1000 Hz/ppb, with a factor from 3 to 5 of uncertainties, mostly due to the uncertainties in knowing the amounts of added NO. With this, the RO2 concentration in the reactor was estimated between 1 X 10(10) and 1 X 10(12) molecules cm(-3). When adding a second radical species to the reactor, the kinetics of the cross-reaction could be studied directly from the decay of the first radical. The time-evolution of two and sometimes three different RO2 was followed simultaneously, as the CH3O2 produced in further reaction steps was also detected in some systems. The rate coefficients obtained are (in molecule(-1) cm(3) s(-1)): KCH3O2+CH3C(O)O2 = 1.2 X 10(-11), KCH3O2+t-butylO2 = 3.0 X 10(-15), Kc-hexylO2+CH3O2 = 1.2 X 10(-13), Kt-butylO2+CH3C(O)O2 = 3.7 X 10(-14), and Kc-hexylO2+t-butylO2 =1.5 X 10(-15). In spite of their good comparison with the literature and good reproducibility, large uncertainties (x5/5) are recommended on these results because of those in the detection sensitivities. This work is a first illustration of the potential applications of this technique for the investigation of organic radicals in laboratory and in more complex systems.