High frequency pure rotational transitions of the simplest Criegee intermediate, methylene peroxide (CH2OO), have been measured up to 1 THz. The data provide the most accurate spectroscopic parameters for the ground vibrational state. The molecule was produced in a flow cell with a DC discharge of CH2I2, O2 and Ar. Doppler limited measurements were made in the frequency ranges 220–320, 580–680 and 970–1080 GHz. A total of 211 transitions were measured and added to four transitions measured in the microwave to obtain a fit up to Jmax = 49 and K max = 14 of the a-dipole spectrum.