We present a new class of EUV antimony carboxylate photoresists with enhanced reactivity and contrast, through the substitution of heteroatoms into the carboxylate. The lithographic performance of (C6H5)3Sb(O2CCH2X)2 photoresists in which X = methoxy, bromine or iodine is presented. The addition of iodine to the photoresist greatly improves dissolution contrast. Utilizing in-situ mass spectrometry, we show how the identity and degree of volatile photoproducts created during EUV exposure change with the composition of the heteroatom.