The petrochemical industry employs assets subject to temporal and site specificity. The OPEC oil price shocks of the 1970s made it difficult to write contracts covering business dealings in the industry. I use this production and economic setting as a natural experiment to test transaction cost theory. In support of the theory, I find that input price uncertainty in the 1970s positively affected the extent of vertical integration by firms into input stages. Moreover, the positive reaction of vertical integration to price uncertainty mainly occurs in transactions subject to asset specificity. I also examine price controls and market power as alternative explanations for vertical integration in the industry, but fail to find support for these hypotheses.