Inter-organizational business processes often involve contracting. ICT solutions for contracting processes must offer high flexibility in changing the structure of the contracting process. This can be achieved by ‘process-aware’ software components which are configured by an explicit model of the contracting process: the contracting workflow. However, the design of a contracting workflow from scratch is a complex task. We propose a solution in which contracting workflows are composed from standard building blocks and show that protocol patterns for business transaction protocols are a necessity for making these standard building blocks available. Finally, we propose a number of protocol patterns for the negotiation phase in a transaction.