We propose UTML as a high level transaction modeling language to facilitate the complex Web transaction design process. Web transactions may be complex, composed of several sub-transactions and they may access resources with diverse behavior and interfaces like legacy systems and databases. They may also have complex semantics. Thus, transaction design methodologies and tools need to be very flexible, allowing for designing Web applications from scratch (top-down design), as well as using existing systems or services to compose new applications which offer added-value services (bottom-up design) to the user. UTML is based on a transaction meta-model which can describe, in a flexible and extensible manner, most of the known transaction models as well as new ones according to the application's requirements. It provides modeling for transactions that incorporate different behavioral patterns, and it is capable of describing activities with weaker transactional semantics that do not have all the ACID properties. Unlike other models, it can be used to synthesize new transactions from pre-existing transaction systems (like legacy systems), with diverse transactional semantics. UTML provides a rich notation to visualize the transaction design process. This notation has been built on top of UML using its extension mechanisms.