In the discipline of systems engineering, the flexibility of a system is considered its ability to respond to change. However, Transaction Processing Systems (TPS) are left to be inflexible up to a certain extent to avoid non-standard operations. This leads to the dilemma that incorporation of more Functional Requirements (FR) into TPS could cause systems to become more inflexible, which in turn could affect the usage of, for instance, online airline reservation systems. We conducted a survey in order to examine the physiological, cognitive, social experiences and perceptions of a selected group of people who have booked their tickets either through travel agents or through online reservation systems. The findings indicate that flexibility has a significant impact on the perceived usability of online reservation systems, much more than functional requirements. © 2010 IEEE.