Tamer Çetin is a visiting lecturer at the Northeastern University, Boston. He is grateful to Christopher Brown and two anonymous referees for their valuable comments. This research was supported in the scope of Project No. 2016-02-01- KAP01 by the Division of Research of Yildiz Technical University (Turkey), when Çetin was an associate professor at that university. The earlier versions of this article were presented at the XVII April International Academic Conference on Economic and Social Development, held at the Higher School of Economics in Moscow (Russia) in 2016 and at the Fourth European Academic Research Conference on Global Business, Economics, Finance and Banking (EAR16 Swiss Conference) in Zurich (Switzerland) the same year. The author is grateful to the participants for their valuable comments. The responsibility for any errors remains with the author. 1137 ©2017, Journal of Economic Issues / Association for Evolutionary Economics JOURNAL OF ECONOMIC ISSUES Vol. LI No. 4 December 2017 DOI 10.1080/00213624.2017.1391597 Toward a Wider Analysis of Market Definition: Theory and Evidence from the Turkish Telecommunications Industry Tamer Çetin Abstract: This article develops a novel approach to the analysis of market definition. The article empirically investigates the main components of market definition, such as market power, substitution, and the simultaneous interaction between the demand and its determinants for the fixed and mobile voice services. To this end, I employ the multivariate Johansen cointegration methodology and analyze the Turkish voice telecommunications industry. While the VECM analysis confirms the presence of a simultaneous long-term causality relationship among the variables, the Johansen normalization results reveal demand elasticities that enable the analysis of market power and fixed-to-mobile substitution. My findings suggest that the fixed and mobile voice services, which are traditionally viewed as separate markets, can be defined as products that compete in the single market. Keywords: cointegration methodology, (de)regulation, fixed-mobile substitution, market definition, market power JEL Classification Codes: C32, L43, L51, L96, K21 Market definition has long been a very controversial field in antitrust cases (Baker 2007; Kaplow 2014; Massey 2000; Tardiff and Weisman 2009; Werden 2014). Even though different approaches to market definition have been presented so far, consensus has not been achieved on which approach is the best to define a market. The aim of market definition is to understand market power and its anticompetitive effects. Thus, the Lerner index, the market shares of firms, and market concentration have been used as traditional tools to measure market power. However, these approaches suffer from some shortcomings when used to analyze market definition