The hydroxymethyl peroxy (HMOO) radical is a radical product from the oxidation of non-methane hydrocarbons. The present study provides theoretical prediction of critical spectroscopic features of this radical that should aid in its experimental characterization. Structure, rotational constants, and harmonic frequencies are presented for the ground and first excited electronic states of HMOO. The adiabatic transition energy for the Ã←X̃ process is 7360 cm(-1), suggesting that this transition, occurring in the mid to near infrared, is the most promising candidate for observing the radical spectroscopically. The band origin of the Ã←X̃ transition of HMOO is calibrated and benchmarked with the corresponding state of the HOO radical, which is experimentally and theoretically well characterized.