The guest-host dynamics of NO2 embedded in He,, droplets have been examined by recording depletion spectra of mass spectrometer signals at m/z values of 8 (HC2+), 30 (NO+), and 46 (NO2+) throughout the wavelength range 340-402 nm. At energies above the A/X conical intersection, gas-phase NO2 is known to exhibit quantum chaos, and it is also known that deactivation of embedded NO2 by the helium host is efficient in this regime. Above the gas-phase dissociation threshold (DO) it is shown that there is no net unimolecular decomposition all the way up to D-0 + 4300 cm(-1). At the upper end of this range, gas-phase NO2 decomposes with rate coefficients whose values are similar to5 x 101(2) s(-1), which is expected to be larger than the deactivation rates in liquid helium. The deactivation rate in the quantum-chaotic region 17 700-18 300 cm(-1) was found previously to be similar to1.4 x 10(12) s(-1), and it is expected that this will increase at yet higher energies, but not exceed 5 x 10(12) s(-1). To within the experimental uncertainty, it is found that reaction products do not leave the droplets. This is attributed to efficient relaxation and (at the highest energies examined) recombination within the droplets. On the basis of these results, it is concluded that small polyatomics embedded in He-n droplets that have values of similar to10(4) or larger will not undergo net unimolecular reaction if the gas-phase pathway is barrierless, with two possible exceptions: (i) if one or both of the products has a positive chemical potential and (ii) if the scattering cross section of the product(s) with helium is small.