Microfeature workpieces having conductive vias formed by chemically reactive processes, and associated systems and methods are disclosed. A method in accordance with one embodiment includes disposing a conductive lining on walls of a via in a microfeature workpiece, so that a space is located between opposing portions of the lining facing toward each other from opposing portions of the wall. The method can further include chemically reacting the lining with a reactive material to form a chemical compound from a constituent of the reactive material and a constituent of the lining. The method can still further include at least partially filling the space with the compound. In particular embodiments, the conductive lining includes copper, the reactive material includes sulfur hexafluoride, and the chemical compound that at least partially fills the space in the via includes copper sulfide.