Transaction cost theory and application tells us that when buyers and sellers in a market incur transaction costs, intermediaries may become involved. Specifically, intermediaries influence the cause of the transaction costs to buyers and sellers such that transaction costs are reduced. In this paper we assess if and how this occurs for a number of case study government created and private emergent intermediaries in Australian environmental offset markets. We find that the causes of transaction costs to buyers and sellers in offset markets - asset specificity, uncertainty and transaction frequency are influenced downwards by intermediaries. The degree of influence depends on the nature of the good traded in the offset market. We also assessed if the public intermediaries studied were operating in the offset markets to reduce the incidence of probity hazard (poor transactions) from private intermediaries. We found that this was not the case.