At -90 degrees C in acetone, a stable hydroperoxo complex [(BA)(CuOOH)-O-II](+) (2) (BA, a tetradentate N-4 ligand possessing a pendant -N(H)CH2C6H5 group) is generated by reacting [(BA)Cu-II(CH3COCH3)](2+) with only 1 equiv of H2O2/Et3N. The exceptional stability of 2 is ascribed to internal H-bonding. Species 2 is also generated in a manner not previously known in copper chemistry, by adding 1.5 equiv of H2O2 (no base) to the cuprous complex [(BA)Cu-I](+). The broad implications for this finding are discussed. Species 2 slowly converts to a mu-1,2-peroxodicopper(II) analogue (3) characterized by UV-vis and resonance Raman spectroscopies. Unlike a close analogue not possessing internal H-bonding, 2 affords no oxidative reactivity with internal or external substrates. However, 2 can be protonated to release H2O2, but only with HClO4, while 1 equiv Et3N restores 2.