The increasing degree of computerization over the past years had led to higher amounts of sedentary time and time spent in static postures, which can be associated with higher risks of chronic diseases, obesity and even mortality. To counteract this negative developments a novel workplace conception called "Active Office" consisting of a standing and a sitting desk as well as an active chair is going to be developed. This new working place concept should motivate users to more physical activity by changing working posture and position whenever desired. An important part for the objective evaluation of this concept is the possibility to track the user's physical activity during several working days in comparison to a standard working place configuration. In this paper two prototypes of instrumented chairs are introduced, which allow to measure and quantify the sitting behavior of different persons. The devices consist of a smart sensor platform integrating sensors for posture measurement as well as the communication via a wireless protocol for data acquisition. The two prototypes were evaluated by means of a user study, where the sitting behavior of different users was recorded during several working days. It turned out that the instrumented office chairs are able to detect the user's physical activity within different working place configurations which provides a valuable basis for the further development and evaluation of the Active Office concept.