Ancillary services costs were included within the energy price in the first electricity market models. In a decentralized electricity market, it is conceive to have a separate market for energy and each one of the Ancillary Services. However, when the problem is solved jointly it seems to be a better alternative to optimize resources. This paper presents a unified formulation for solving the economic dispatch and reserve scheduling problems in electricity markets, in order to investigate the global efficiency of energy and reserve markets. The formulation allows generation and distribution companies to offer in the market reserve services. Concluding remarks are offered about the results obtained, which proves in a detailed way that these operated–simultaneously–markets provides more economical benefits for generation or distribution companies.