This paper provides a diversification explanation in order understand the development of PTAs in Southeast Asia. I argue that an important reason why ASEAN states participate in PTAs has been to diversify existing trade ties and to reduce overdependence on a narrow range of export markets. Southeast Asian countries have formed PTAs with markets with which they had weak or unexplored economic relations, as demonstrated by three case analyses: the ASEAN Free Trade Area (AFTA), the ASEAN-China Free Trade Agreement (ACFTA) and the ASEAN-Japan Comprehensive Economic Partnership Agreement (AJCEP). To maximise the economic gains and the diversification effects of PTA participation, ASEAN countries have pursued a strategy of strengthening economic unity while keeping external economic linkages as diversified as possible. Although East Asia, and especially China, was an important alternative market to reduce ASEAN’s dependence on trade with America, ASEAN countries have also pursued PTAs with a number of other trading partners. This paper explains how PTAs have helped ASEAN states to develop more policy autonomy in their trading environment.