Experiments using pulsed Fourier-transform microwave spectroscopy have been performed to probe the reaction between the methyl substituted Criegee intermediate, CH3CHOO, and hydrogen chloride. Two conformers of the product derived from the insertion of HCl within CH3CHOO, chloroethyl hydroperoxide, formed in the discharged plasma of a CH3CHI2/O2/HCl gas mixture, were identified through their experimentally determined rotational and quadrupole coupling constants. The relative abundance of the detected species indicates that the reaction of the anti-CH3CHOO conformer is faster than that of the syn-CH3CHOO conformer. The results presented in this work constitute the first experimental information about the reaction between CH3CHOO and HCl.