We have recorded several rovibronic bands of C H3 OO and C D3 OO in their Ã X̃ transitions in the range of 1.18-1.40 μm with the cavity ringdown technique. While the electronic origins for these species have been reported previously, many newly observed rovibronic bands are described here. The experimental vibrational frequencies (given as in the unit cm-1 in this paper) for the COO bending (8) and COO symmetric stretching (7) modes in the Ã state are 378 and 887 cm-1 for C H3 OO, and 348 and 824 cm-1 for C D3 OO, respectively. In addition, two other vibrational frequencies were observed for the Ã state of C D3 OO, namely, 5 (954 cm-1) and 6 (971 cm-1). These experimental vibrational frequencies for the Ã state of both C H3 OO and C D3 OO are in good agreement with predictions from quantum-chemical calculations at the UB3LYP/aug-cc-pVTZ level. The enhanced activity of the 5 vibrational mode in C D3 OO is rationalized by mode mixing with the 7 mode, as supported by calculations of multidimensional Franck-Condon factors. In addition, many hot bands involving the methyl torsional mode (12) are observed for both normal and deuterated methyl peroxy. These bands include the "typical" sequence transitions and some "atypical" ones due to the nature of the eigenvalues and eigenfunctions which are a consequence of the low, but very different, torsional barriers in the X̃ and Ã states. In addition, the 1222 band in C H3 OO and the 1233 band in C D3 OO show quite different structures than the origin bands, an effect which results from tunneling splittings comparable to the rotational contour.