High level ab initio calculations and multiconfigurational methods have been used to characterize the equilibrium structures, vibrational frequencies, enthalpies of formation, and UV-vis spectra of alkyl peroxynitrates R-OONO(2) (R = H, CH(3), C(2)H(5), C(3)H(7)). Excellent agreement with the experimental enthalpy of formation is obtained for HOONO(2) warranting similar accuracy for the rest of compounds for which values are inexistent or measured indirectly. The spectra obtained by MS-CASPT2/CASSCF calculations are very similar in all the species, showing a broad band below 200 nm with a shoulder due to pipi* transitions and a tail at approximately 250 nm due to weak npi* transitions on the NO(2) group.