The invention provides efficient, inexpensive, and environmental friendly catalysts and catalyst systems. The catalysts can be used to catalyze esterification and/or transesterification reactions, for example, for the preparation of biodiesel. Kiln dust, such as cement kiln dust (CKD) or lime kiln dust (LKD) can be used to convert a variety of feedstock acids and/or esters to biodiesel in high yield under mild conditions. The CKD and LKD catalyst systems are recyclable esterification or transesterification catalysts that can be used to prepare biodiesel, such as methyl soyate, from various feedstocks, including vegetable oils and animal fats.