Did Thailand’s multiple parties and factions influence cabinet and coalition durability in the period 1979 to 2001? If so, which one – parties or factions – was the more significant? Taking a Transaction Costs Analysis approach, this article addresses these questions and argues that intra-party factions, as the building blocks of Thai parliamentary politics, have been more important than parties, such that each additional faction in a cabinet triggers a reduction in the longevity of prime ministerial terms and cabinets while affecting the durability of coalitions. Furthermore, while factions tend to shorten parliamentary and cabinet terms, they have the opposite effect on coalition terms. The study suggests that where parties are less cohesive, informal institutions within parties are of considerable importance.