The extreme durability of polyethylene terephthalate (PET) debris has rendered it a long-term environmental burden. At the same time， current recycling efforts still lack sustainability. Two recently discovered bacterial enzymes that specifically degrade PET represent a promising solution. First， Ideonella sakaiensis PETase， a structurally well-characterized consensus α/β-hydrolase fold enzyme， converts PET to mono-(2-hydroxyethyl) terephthalate (MHET). MHETase， the second key enzyme， hydrolyzes MHET to the PET educts terephthalate and ethylene glycol. Here， we report the crystal structures of active ligand-free MHETase and MHETase bound to a nonhydrolyzable MHET analog. MHETase， which is reminiscent of feruloyl esterases， possesses a classic α/β-hydrolase domain and a lid domain conferring substrate specificity. In the light of structure-based mapping of the active site， activity assays， mutagenesis studies and a first structure-guided alteration of substrate specificity towards bis-(2-hydroxyethyl) terephthalate (BHET) reported here， we anticipate MHETase to be a valuable resource to further advance enzymatic plastic degradation.