The paper provides a model that explains mixed land use patterns in urban areas. The paper contains a proof of the ex-istence of competitive market equilibrium in a market for land in a small zone where more than one land use may be present. The model includes the possibility that external effects (both negative and positive) exist among the land uses. Brouwer’s fixed point theorem is used for the proof. Conditions are derived in which mixed land use is an equilibrium pattern in a monocentric city.