Since service production and consumption are inseparable, many service firms sell their capacity in advance. Consequently, the question is whether it is optimal to sell more or less in advance, and if so, at what price and quantity. This paper examines the optimality and practice of advanced sale of service capacity under duopolistic competition.Using a game theoretic approach, the paper provides a possible explanation as to why, despite sophisticated practices of advanced sale in many service industries, there are still many other service firms that commonly do not sell in advance. It demonstrates that competition and market price sensitivity to a firm's capacity are two factors that can impact not only on a firm's decision to sell in advance, but on its pricing and capacity allocation decisions as well.